Hello David, it is a use case that personally never crossed my mind but you are right, it’s not possible at the moment and we don’t have any outfit that would be just shirtless. We don’t have an option to remove the top clothing and we don’t have an outfit that would be “shirtless”.
For now if you want to achieve that you would need to upload it as a Hero Character, although users selecting it would lose all customization options.
I will share your feedback with the team, if you want to ellaborate more on the use you plan for the shirtless avatar I can add more context to the feedback list.