I’m drawing clothing buttons on a shirt, but i’d like to do this in a different subtool - can I draw on background subtools?
No. For performance reasons the other SubTools are only visible. They’re not typically directly accessible.
What you could do is clone the SubTool that you wish to draw on. Assign it all to a single polygroup. Then you could use your IM or IMM brush to draw your buttons. Once drawn, hide the base model’s polygroup and then delete its points. Now you’ll have the buttons – and only the buttons – as a separate SubTool.