I kinda solved this problem a few years ago. I made a duplicate
of my gig database then transposed all the tunes to the horn key
in the second database.
Then printed the tunes and made a gig book for him. I know this
is old school but it works. My partner jpeg'd his book put it on a iPad
with a foot switch that changes pages for him. Everything is running smooth
The person who controls the Box should have the tunes in there concert key
and the other should transpose.