Rug,
I think you're looking for the acad.mns file. This is the file which contains the definitions of all the menus and pop ups.
I've customised my snap menus by removing the crap snaps that I don't use and adding my most frequently used 'hard' commands
(ie commands which are buried in long menu chains or which require multiple inputs; for example I have a CPS command which purges the drawing of all objects, audits the drawing, resets the view, zooms all, saves and closes the file)