Aside from cutting down the tree (more on that later) I see nothing wrong with the FS actions. Cairns, signs, and blazes are simply not allowed on unofficial trails, Wilderness or not. People were quite upset about the bootleg trail that was cut to the Peak above the Nubble and most people here are strongly against leaving flagging up on bushwacks. The rangers are simply following the rules that were set above them. Putting up the sign about not doing unauthorized trailwork is common sense and the FS has done it elsewhere. It sounds like standard policy and I don't find it intrusive in the least. There's a link somewhere to the WODC page where a similar sign was posted near Whiteface.
Now you can argue about why now, or the enthusiasm in which they take their jobs, or priorities based on other needs in the forest, or the heavy use of the herd paths, but I find it difficult to fault the rangers for doing their jobs. Argue higher up, and work on getting the Owls Head path made official and this issue will go away. Antagonizing the rangers seems counter-productive, we could be building a nice alliance here. By all means, make yourself heard but please try to keep this in context.
It's hard to tell from the photo where or why the tree (was there more than one?) got cut down. I find it hard to come up with a valid reason, but I'd like to hear more about it before deciding that it's a huge deal. There's no problem cutting down trees in the Wilderness Area, it's done for trail work. Since this isn't a trail, I'm at a loss as to why any trees needed to be cut. While a bit ugly, I have no problems with scraping the blazes off the trees. I've done it when closing old trails, the trees seem to recover just fine.
The best solution, IMO, would be to work with the 4000'er Committee, the AMC, and the WMNF folks to come up with a maintainence plan for this route/trail and help self-police the hiking community. Putting up signs, cairns, and blazes, while making people feel good in the short term, will not help lead to a long term solution that makes everyone happy.
OK, that's a bit long winded and preachy, so I'll go think about hiking for a bit.
-dave-