This is not possible to do, while maintaining the way xmmv works right now. The behavior you expect involves that xmmv runs as a background process, while grabbing the shortcut Mod4+click. By doing so, it could detect mouse press and mouse release events, and then move the window accordingly.

However, the way it works now is that you only run it when you want a specific window to move. So you must have a way to spawn the process. Once the process is started, it must grab user input to detect the next mouse release event.
There is no way (that I'm aware of) that would make the following work:

1. Press Mod4 and hold click down
2. Spawn xmmv
3. Have xmmv detect mouse release

We did some testing, and xmmv just doesn't see the mouse release, because it is only reported to the program that spawned it (usually sxhkd or xbindkeys).

tl;dr: nope, impossible. Use a dedicated process that would run at all times to do it, like swm or glazier.