Is there any textual basis for translating 'dedikaiwtai' as "acquitted" in Romans 6:7?
Yes, I believe so...
As Dottard has previously stated:
The Greek word in Rom 6:7 is δεδικαίωται which is the perfect indicative verb from the root verb δικαιόω = make righteous, defend the cause of, plead for the righteousness (innocence) of, acquit, justify (Strongs).
But notice the word "free" isn't a part of this definition, but the word acquit is.
In Thayer's Greek Lexicon's entry for STRONGS NT 1344: δικαιόω, there is a section that relates both concepts of acquittal and freedom.
3.τινα, to declare, pronounce, one to be just, righteous, or such as he ought to be, (cf. ὁμοιόω to declare to be like, liken, i. e. compare; ὁσιόω, Wis. 6:11; ἀξιόω, which never means to make worthy, but to judge worthy, to declare worthy, to treat as worthy; see also κοινόω, 2 b.);
a. with the negative idea predominant, to declare guiltless one accused or who may be accused, acquitted of a charge or reproach, (Deuteronomy 25:1; Sir. 13:22 (21), etc.; an unjust judge is said δικαιοῦν τόν ἀσεβῆ in Exodus 23:7; Isaiah 5:23): ἑαυτόν, Luke 10:29; passive οὐ δεδικαίωμαι, namely, with God, 1 Corinthians 4:4; pregnantly with ἀπό τῶν ἁμαρτιῶν added, to be declared innocent and therefore to be absolved from the charge of sins (cf. Buttmann, 322 (277)), Acts 13:38 (39) (so ἀπό ἁμαρτίας, Sir. 26:29; simply, to be absolved, namely, from the payment of a vow, Sir. 18:22 (21)); hence, figuratively, by a usage not met with elsewhere, to be freed, ἀπό τῆς ἁμαρτίας, from its dominion, Romans 6:7, where cf. Fritzsche or ((less fully) Meyer).
What is interesting is that δικαιόω does not mean free except perhaps once in Romans 6:7 and only there by applying a figurative meaning.
In reality, δεδικαίωται doesn't literally mean free, but, in context, δεδικαίωται ἀπὸ τῆς ἁμαρτίας, essentially means "freed from sin" because the person is righteous/guiltless/innocent and has been pronounced to be so. Freedom here is but the necessary implication of the legal consequences of having been declared righteous.
Now we may be missing the point if we latch on to the meaning of "freed" without regard to the underlying meaning of "pronounced to be righteous." For example, it's easy to focus only on the phrase 'freed from sin' and derive the meaning that one (really anyone) who has died is free from sinning or is free from the temptation of The Sin as a practical implication of being dead.
While this idea may be, in a sense, true (since, in fact, a dead person doesn't commit very many acts to begin with), this interpretation has nothing to do with whether that same person has actually been declared righteous away from the Sin and the legal implication of that declaration. It thus fails to account specifically for what the text actually says. Being careful to keep in mind the underlying meaning of the word δεδικαίωται could prevent the advocacy of such an interpretation which is based on the concept of freedom without regard to the declaration of righteousness.
Does Romans 6:7 suggest that a person is acquitted from their sin at death?
I don't think so...
First, Romans 6:7 doesn't grammatically refer to personal sins but figuratively to an antagonistic entity, "the Sin."
The verse can be literally rendered as follows:
ὁ γὰρ ἀποθανὼν δεδικαίωται ἀπὸ τῆς ἁμαρτίας.
For one having died has been declared righteous away from the Sin
For he having died has been declared righteous away from the Sin
Typically this is translated, akin to the former option, in a proverbial sense as if discussing a basic fact about anyone who has died. Translating it the latter way would contextually make Jesus the individual who, having died, was declared righteous away from The Sin.
That said, the latter view should be preferred and the former discounted, for only Jesus was, having died, directly declared to be righteous - visually vindicated - publicly pronounced the righteous son of God before all.
Acts 17:30-31 30 "Therefore, having overlooked the times of ignorance, God now commands all people everywhere to repent, 31 because he has set a day when he is going to judge the world in righteousness by the man he has appointed. He has provided proof of this to everyone by raising him from the dead."
Romans 1:4 and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord,
So it is, in fact, through the resurrection that Jesus was declared righteous away from the Sin—vindicated away from all the sin that he bore in his body on the tree (1 Peter 2:24), by being raised up from the domain of death. Because Jesus did never sin, the Sin could not claim him, and so death could not keep him. Jesus was vindicated and declared righteous and blameless through the resurrection because he was actually completely righteous and blameless (1 Peter 2:22, Phil 2:8-9, Heb 5:7, ).
22 He committed no sin, neither was deceit found in his mouth.
-1 Peter 2:22
7 In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to him who was able to save him from death, and he was heard because of his reverence
8 And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. 9 Therefore God has highly exalted him and bestowed on him the name that is above every name,
Conclusion: Jesus is the Way
Interpreting verse 7 with Jesus as the subject flows much better than the alternative interpretation within the immediate and surrounding context which is all about Jesus's death and resurrection and how good an idea it is that we are united with his death because of his resurrection. It's not about the positive spiritual benefits people, in general, experience when they die. Only Jesus is the Way. The solution against Sin is not in ourselves.
Again, only Jesus Christ has been proven righteous after having died... Well, only Jesus Christ and those who, in a spiritual sense, have been united with him in his death.
5 For if we have become united with him in the likeness of his death, we will also be part of his resurrection;
6 knowing this, that our old self was crucified with him, that the body of sin might be done away with, so that we would no longer be in bondage to sin [the Sin].
For he, having died, has been declared righteous [by his ressurection] away from the Sin
8 Now if we have died with Christ, we believe that we will also live with him.
9 We know that Christ, being raised from the dead, will never die again; death no longer has dominion over him.
10 For the death he died he died to sin [the Sin], once for all, but the life he lives he lives to God.