Some more tips for those trying this.

Regarding button usage, This wiki link lists all the available keys in WoW. Keep in mind that once you use a key for a hidden macro, you're using all of its modifiers too.
It shows that you can't use shift+numpad buttons, which I haven't tested myself, but if thats true leaves a set number of buttons that could conceivable run out.

I did, however, come across this today while searching. If you want to use the P button as a hidden macro, but still have P open up the spellbook, try putting this at the end of the macro.

/stopmacro [mod]
/click SpellbookMicroButton

This should make it so if you just press P, it will still open up the spellbook. Unfortunatly you can't just leave P binded in the key bindings menu, otherwise it won't call the macro when needed. You can use this for a lot of the other menus ingame, thereby freeing lots of buttons for macros.

Also, I set up a google spreadsheet to help keep a list of Ability --> Keybind --> Hidden macro, and which characters have which modifier combinations. Helped me a ton while troubleshooting.

My WoW computer still isn't working, so I'm unable to test this, please tell me if it doesn't work for some reason.

Oh, and.... Bump!