If you are trying to dove-tail piety to True20, just add the score in. It was just an add on to the d20 system; it was never fully integrated. What aspects of piety pose a special problem to True20? (Forgive me, I'm still trying to adjust to the True20 system, but I can't think of any problems with the piety mechanic that would make it difficult to just add it in).
Piety is a lot more complex than conviction, but in a way that makes it rather ideal for more religiously-conceived campaigns (it was created for Testament). It's about more than just following one or two general ideas about what your character likes to do-- it's about an entire code of conduct, and the penalties and merits received based on how well you follow your code are more varied and larger-scale than conviction.