I love hot melt for many reasons. I even tried using it on picture
frames like you. Since how a frame looks has a lot to do with how
closed and neat the miters are, I would tend to stay away from hot
melt. Yes, you can heat the frame materials and it will probably do
OK. If you aren't going to use something to reinforce the joint, then
I might try hot stuff or some other brand of medium thick
cyanoacrylate. It is quick and pretty strong. And you don't have the
problem of the adhesive potentially creating a gap No glue is going to
be real strong on an unreinforced miter.