"Biblical proof" or "scriptural proof" is a misnomer; the Bible makes claims. Proof or evidence must be sought out if one seeks to validate (or invalidate) those claims.
In any case, this level of navel-gazing is fascinating to me. As if we're supposed to be concerned that god, when he caused a magical conception, made sure not to break any of the laws that he had given to the Israelites. The method is also of no real interest, save for the fact that it makes this claim impossible to corroborate. God could have waited for Joseph and Mary to wed and bed, and then replaced the fertile egg with one that contained his godly essence. He could have waited until their child was of a certain age, then transferred the child's soul to heaven and infused the body with his own intellect. And so on.
It's academic, in light of what god wanted to accomplish. If the important thing was to preach the word of the new covenant and then sacrifice his life as a ransom for humanity, that could have been done in any of a number of ways. After all, the whole thing was being set up so that god could pay a debt to himself on behalf of humanity. He could have accomplished the very same thing by simply saying "you're all off the hook." The flair for the (gruesomely) dramatic made for a bit of drama (and a very graphic film by Mel Gibson) but was unnecessary.