You can assign the LMB to move the player forward, you wouldn't want to move the mouse forward to do it as it would get tedious but player forward is fine by pressing and still look/move around with mouse. It's been done before and works pretty well. In fact I found it to be a nice intuitive experience ideal for adventure type games.
Of course recent updates might have changed the ability to do it, still I would check the syntax list to see what could be done now
Edit* Found the old script for using RMB (I wanted to use a weapon too) and it still works just fine, you can adapt this to suit yourself
Place it in a trigger zone anywhere in your level.
;Artificial Intelligence Script
desc = Plr In Zone
;Triggers
:state=0,always:jumpkey=0,crouchkey=0,arrowkeys=0,state=1
:state=1,mouseclick=0:plrforcemove=0
:state=1,mouseclick=2:plrforcemove=100,state=2
:state=2,mouseclick=0:state=1
;End of Script
