This is a wild request, and probably not possible to do, but that's why I ask these things!
One of the most criminally underutilized Touch Bar interaction methods is Apple's own UI Select-driven mouse interface. To my knowledge, the only place where a user can drag their mouse off the bottom of the screen and onto the Touch Bar is when customizing Touch Bar buttons through the macOS system overlay.
But, well, this would actually be a great thing to be able to do all the time - treat the Touch Bar like a true second screen and let me move my mouse down onto it.
I don't think the cursor should actually show up; similar to the built-in interface, or Apple TV, it would use a highlight selection-driven approach that hides the actual cursor.
Again, no clue if it's possible with BTT, but I've tried about three times now to move my mouse down to the Touch Bar and it's starting to drive me nutso.