Hello and welcome to DC!
The truth is that this is possible, but I hadn't thought about it.
It makes sense, because using the middle button for this can be annoying.
I'll add it to the todo list, and meanwhile, you can just add the windows to the exceptions.
PS: gridmove includes exceptions for middle mouse button specifically, but those are hardcoded, so, if you could make me a small list of the windows that cause you trouble, I could include those in the next release.