One place that might work is Puffer's Pond in Amherst. There's a bit of a picnic-like area that just might have enough access. Check out this trip:
http://www.mafishfinder.com/view_trip.php?trip_id=3208. The spot where the trip is marked is the area I am talking about. You'll probably have to scout it before going since I haven't been there in the last year or so. The pond is stocked with trout.
Failing that, I guess you'd need to look for something like a boat ramp or a bridge or something like that. I don't know how all-terrain wheelchairs are these days. There must be some marinas along the connecticut river that would let a couple of guys spend a little time fishing there.
The only place I know of all the way up by Greenfield is the Deerfield river out by Shelburne Falls, and that doesn't really have any decent access. Your best bet probably is the CT river, since you have a stretch running right through up there.