I did a winter hike of the Western Seneca peak just a few months ago. No sign of restricted access. I go in from Lost Nation Road. I also snowmobiled the trail that goes on Madison Brook road across the base of Seneca in March. So far so good, but it could always change. I have done all 115 peaks in VT above 3000' (110 3k; 5 4k) and I must say in my opinion East Seneca is the toughest in all of Vermont. Just FYI. Super thick conifers and lots of blowdown, especially in the col between the Senecas.
It's a great mountain and one of the most remote places you can possibly go in Vermont.