Sorry. I missed the intent of your question which was focused on multi-part songs and not the one-press variety. Yeah, it is a pain to have to export each part of a song, do the edits in a DAW and then add each MIDI part back to the BBM song. I think this is why some users would rather edit the drum kits instead.
To expand upon your thoughts just a little further, I wonder if there’s great enough user demand for a BBM utility where a user can batch the entire process:
Export to MIDI files all parts of a song
Select the instruments to be edited across all of the exported MIDI parts
Edit the velocities for all the selected instruments
Add edited parts back to their respective locations in the BBM song
It would even be better though if the BBM MIDI Editor were up to that task where a user had the option of selecting the drum instrument(s) across a range of parts within a song or a range of songs within a folder or a global change across all songs in all folders for say, toms, cymbals, etc and then editing velocities by percentage. Example reduce all crash cymbal velocities by 35%, increase all tambourine velocities and cowbell by 20%.
Wishful thinking, I know and for now, I’d settle for an updated BBM with bug and performance fixes as well as a few features such as autopilot. So far just crickets and vaporware :(.