There have been many past threads on this board about abandoned trails in the Whites. Some of these closed trails have been posted by the USFS with threats of $1000's in fines should you be caught "re-opening" (i.e., maintaining) them. Ironically, some of these closed trails are now being illegally maintained better than the USFS and the AMC is maintaining many of their open trails. The USFS refuses to take the time and effort to involve volunteer adopters for their trails and the AMC does not keep very good track of the performance by their volunteer adopters. The DOC, RMC, and WOC seem to do a better job with their more limited trail systems. The AMC does send their small professional crew through all of their trails in the late spring to remove blown down trees.
So, it seems to me a logical approach for those interested in volunteering to maintain trails would be simply to carry a pair of hedge cutters or even a small pair of snippers with you while hiking on officially open trails and do some brushing (called "standardizing" by the pro's) whenever you stop for a rest. The only real rule is to snip off the branches close to the trunk so that you do leave long, sharp, pointed protrusions to impale hikers. Standardizing also is defined in such a way that one can hike a trail with a 4 ft x 8 ft sheet of plywood carried upright on a packboard without obstruction by branches. Some times it makes more sense to remove an entire sapling encroaching on a trail rather than snipping most of its branches. One can also carry a small fold-up pruning saw to remove trees that blow down after the professional crews have completed their work in the spring. Finally, cleaning water bars of debris is an important process that needs to be done to prevent trail erosion throughout the hiking season, but especially in the fall and spring; a heavy-duty pulaski works better than lighter-weight hoes for this chore.
I think that "vigilante trail maintenance" is probably the only way that we will save our White Mountain trails from becoming completely overgrown, which is happening faster than ever with global warming and the lack of funding for trail maintenance. As noted in other threads on this board, most of the USFS parking fees (Fee Demo) go into office supplies and equipment, new vehicles for enforcement, and upgrading trail head facilities, rather than trail work.