Greetings my friend, [Roy]!
My original intuition when reading your email was that the suggestion you mentioned could not be correct in view of how XRISTOS functions in the NT. BDAG confirmed what I already thought, but I'd like to nuance what is found there.
(1) By definition, a verbal adjective is a participle. Hence, suggesting that XRISTOS is a participle in the NT just did not seem right to me. In fact, BDAG points out that while this word was used adjectivally in Gk. tragedy and the LXX (et al.) it is only used as a noun in our literature (i.e., early Christian literature).
(2) BDAG also maintains that XRISTOS (in the NT) is a "personal name ascribed to Jesus, Christ" and was understood thus by certain first-century denizens who were in some way familiar with the figure possibly known to them as XRHSTOS (XRISTOS might have sounded like XRHSTOS to Gentiles). So it's possible that XRISTOS is a personal name.
(3) I would nuance the comments in BDAG by insisting that XRISTOS is actually a quasi-personal name. It functions to delimit referentiality like a personal name does, and perhaps even more forcefully. In the NT, based upon the term's frequency of usage, XRISTOS is almost certainly a title or quasi-personal name for Jesus, God's anointed one. Interestingly, Granville Sharp considered IHSOUS to be a proper name, but thought of XRISTOS as a "noun of personal description, denoting office, rank, title, or the like" (Richard A. Young, 63).
(4) Genitive does not primarily or always mean possession, but I think that the genitive case delimits TWi ONOMATI in Acts 2:38. The genitive is actually the descriptive case. D.A. Black points out that genitives are adjectival per their function; however, these nouns are more emphatic than Greek adjectives. Compare the rendering "sinful body" with "body of sin" (genitive).