Well, I really need this working, so I guess I'll resurrect this with some new things I've found.
First off, lifting the message handling code from the physics behavior and placing it in a fresh copy of a newer version of the SDK doesn't work. The only way I got my own message handling plugin was by clearing out the physics behavior code of everything physics related and using it as a base.
I've noticed David seems to have done the same thing for the bone movement behavior, as it's based on an ancient SDK as well. Unfortunately, when I attempted this I seem to have broken something related to actions, expression, and conditions because they don't work anymore.
Another thing that's closely related. With the current SDK, trying to capture the frame editor crashes Construct when your plugin is a behavior. It doesn't crash when it's a standalone plugin, so I don't know what's up with that.