A few years ago (at my wife's insistence), I started carrying a small sandwich size ziplock bag inside my main TP ziplock.  After doing my thing, the used TP goes into the small bag.  All used TP then gets hauled back to civilization for final disposal.

I have found this method to work well during my last few Quetico trips (including a 12 day trip completed a couple weeks ago) as well as several backpacking trips.  The 12 day trip was the longest I've ever been out without going in for a resupply and all my used TP fit inside the small bag.  This isn't as bad as it might sound as long as you keep the bag sealed shut.  I'm sure the next occupants of our campsites appreciate it.