I haven't tried what you described (using a user-defined function for the SKIP FOR clause in menu items) ever since I ran into a problem with that technique back when I was using FoxPro for Windows 2.6. The user-defined-function would get called for practically every menu pad as well as each bar of pop-ups -- EVERY TIME when ANY portion of the menu got focus. This meant that the menu would seem to lag behind the user (in some cases it was bad enough that you had to hold down the mouse button for nearly a half-second for the menu to register). The more extensive the menu is, the worse the problem. I don't know if the situation is different in VFP or not as I've avoided building menus that way.
As for using GENMENUX -- I agree wholeheartedly. Made a lot of functionality that would've been otherwise rather painful to achieve without it.