The standard emergency treatment for frozen pawls (spinning freewheel) is to use a few zip ties to tie your biggest sprocket to the spokes. It won't ratchet at all then, you're riding kind of a fixed-gear with a derailler, but it'll keep you moving.
I haven't had to do this ever, I've not had frozen pawls, but this is what I've read.
In any case, it always pays to carry some zip ties.