There is an interesting thread in the archives at BPL on this topic.http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=17177&disable_pagination=1
Note the several embedded links in the thread -- they are very good.
It cites some research, and I can't find another thread there where someone did research with the finding that if one peed on the TP before burying it (after using it), that within one month the decay of the paper was almost complete (when redug up and examined).
I wish the NPS would do more research on this. I think were they to do so, the problem is not that significant if people were to always pee on the used TP before burying it, or if not able to pee, at least douse it with water, and stir it a bit in the fecal material before burying.
We're struck with the rule in the interim and another option is to without TP, and that thread above has links to a great tutorial on how to be sanitary and go without TP. It's very easy to get sick if one is not sanitary on this issue.