Have been using it for a while and haven't noticed any problems - well, other than a slight green aura eminating from everyone living in the house, but that means we could get rid of the night lights. Given there's a couple of rinse cycles after the wash cycle, I'm not too worried about it.
I also found a couple of sources for STPP. Curiously enough, it comes in three grades, the highest being food grade, where it's used as a seafood preservative! Will see how that compares to TSP.