Since RegisterHotKey has VK keys for mouse buttons I thought I'd try them. Even if they register without error, nothing happens when you click in combination with a modifier key.
I suspect it's dead otherwise it would be simple for AutoIt3 to use global mouse hotkeys. I know it's done in c++. That leads me to suspect the facility is either totally broken or convoluted enough to be unusable.
So again it looks like if you want global mouse hotkeys ahk is the choice.
I'd prefer coding with something with more scoping structure.. but it's tough to beat it for ease of mouse hotkey creation.
Just thought I'd post in case someone has done it. But, if it was easy the AU3 guys would probably have done it already.