Ah, very cool!
If you can publish the code, a bill-of-materials with links to places to
purchase, and some instructions to put it together, I'm sure quite a few
people will build these. I sure would make one. Some people may also
want to tack one BLE adapter on an actual hardware serial port of a TNC of
some sort, it wouldn't be a difficult modification to the firmware for
someone to do based on your code.
I suppose, if you sold these, a few people would buy them too, until
Kenwood makes a D76 or something with BLE support. Quite a few people
would have some difficulty getting the firmware installed and the kit set
up so there might be some "customer support" questions.
Finally got some time to play some more with a proof-of-concept adapter for TH-D74. It's working reasonably well with RadioMail app for winlink email. Should work the same with APRS.fi.
Best part is that you can make your own!