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.
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!