Having just read the fic, I'm now considering the logistics of this. Given that taking Harry's blood when he's Lily doesn't work for the super polyjuice, I can only imagine that this would end up having Unfortunate Consequences for the 'verse you've created here. Granted, given how much Harry hates James, it's impossible that he would do such a thing at all, but it's still something that is somewhat mind-boggling.
I wonder how surrogacy works? Does an egg removed from Harry's body vanish when the polyjuice wears off? Does it turn into semen as the nearest male counterpart? Does that mean that Harry could accidentally impregnate a surrogate instead of having her carry Lily's baby? The possiblities for crack fic appear to be neverending.
(btw, since I just realized I didn't say it, I really enjoyed reading this. It was a completely novel idea executed very well.)