I think it'd be cute to keep the absolute coordinate system, however create a simple web app which lets you point and click where you want it the brush go. For the Z coordinate, you could have a meter which goes up and down from 0-60 while you hold the mouse, and registers when you let go. (similar to many mini-game mechanics)

It could be done with the current Twitch stream in maybe 2-3 hours of dev work, thanks to Twitch using IRC for the chat backend.

Edit: s/coordinate system/absolute coordinate system/ s/but/however/