Wow I forgot all about this post! I've been putting my water changes into my septic for over 4 years now (I can't believe it's been that long since I asked this question), with no apparent ill effects. I don't do water changes as much as I should, but it seems to be OK.
FWIW - All septic treaments are completely unnecessary, and they can actually do more harm than good. Our local health dept discourages their use becuase they break up the sludge in the tank, and carry it to the field where it will clog it. The sludge is supposed to stay in the tank where it can be periodically pumped out.