The Greek noun ἔκτρωμα occurs three times in the LXX. (Please click here.) In one instance, the wider context appears to be in reference to defiance of the authority of God.
In Numbers 12:12, Miriam is struck with leprosy because of her defiance of the authority of Moses; in fact, Miriam had arrogated herself the title of spokesperson of the Lord on par with Moses (Nu 12:2). When Aaron noticed her with leprosy, he described her as the living dead.
Numbers 12:12 (LXX)
12 μὴ γένηται ὡσεὶ ἴσον θανάτῳ, ὡσεὶ ἔκτρωμα ἐκπορευόμενον ἐκ μήτρας μητρὸς καὶ κατεσθίει τὸ ἥμισυ τῶν σαρκῶν αὐτῆς.
Miriam was not on par with Moses, but on par with someone dead ("...ὡσεὶ ἴσον θανάτῳ..."). In the Masoretic Text we see the Hebrew word is the participial form of מוּת in the Qal (active), but the participle is in the masculine singular. (Please click here for the expanded analysis.) According to Walte and O'Connor (1990), this structure is indicative of the "predicate participle," or what we would call in Modern English the future progressive participle, since the participle occurs in tandem with the Qal imperfect of הָיָה. (Please click here in Google Chrome for best results.) In other words, Aaron was stating that Miriam "will be like the living dead (i.e., leper), whose flesh is comparable to the stillborn child."
Thus the LXX translators used the Greek word ἔκτρωμα to refer to a living human (Miriam), who was like the "living dead." The word ἔκτρωμα was therefore used to refer to the living dead, because the remainder of the verse in Hebrew makes allusion to the stillborn child.
So in summary, in the LXX there is at least one example of the use of ἔκτρωμα to refer to the person who is still living, but "as the living dead."
In conclusion, Paul had made reference to himself as one who was born dead (ἔκτρωμα), since he had defied the authority of Jesus Christ and tried to destroy the church of God (1 Cor 15:9). Like Miriam he took his pharisaical authority to be on par with Moses. After his conversion he was viewed as a leper among the believers of Jesus Christ--that is, they mistrusted and excluded him (Acts 9:26), and, like the leper Miriam who spent time in the wilderness apart from the Congregation of Israel (Nu 12:14-15), Paul too spent time in exile in the wilderness of Arabia (Gal 1:17).
We could therefore translate the verse in question in loose, but amplified form, as follows:
8 and last of all, as to one born a leper, He appeared to me also.
Walke, Bruce K. and M. O'Connor (1990). An Introduction to Biblical Hebrew Syntax. Winona Lake: Eisnebrauns, 628.