The problem with ent_blend and ANM_ADD is that you cant blend single bones as far as I know.
I had the idea of doing it like this:
Dont use ANM_ADD at all, but have a second, invisible model with the same bones and orientation as the player model.
You animate the player models lower half, using ent_blend and stuff, and constantly reset the upper bones.
At the same time, animate the invisible model, read out the bone angles and use ent_bonerotate to kinda "animate" the players arms by hand, using the outread angles of the invisible model.
Dunno if this is clear explained tough.^^