As I think I may have mentioned elsewhere, you can't set a hotkey for any interface item that is not present when ZBrush launches. That means that if you have to switch tools (such as selecting a 3D tool) to make the buttons appear, you'll have no luck. The same applies to dragging buttons to the interface.
One workaround is to create a plugin button that presses the button you want, if it exists. You can assign the hotkey to your plugin button and you won't know the difference. Just be sure that you use an If statement with the [IExists command and all will be well.