Got it soaking in WD40 now and I'll try the hex wrench trick once it penetrates a bit. If that doesn't work I'll try the local smith. Thanks for the suggestions.
But you're right, as far as messing up a receiver, it's not the worst one. It was just annoying because usually I work on my guns to distract myself and relieve stress, not to create it.
The forward assist was moving freely until I started trying to get the roll pin out. Once I started to reverse its travel, I guess the bent part started obstructing, and the forward assist got stuck in the "in" position. After I worked out a little more of the roll pin, I was able to remove it completely, and it's free of the receiver altogether now.