Another idea, although perhaps would require too much compute overhead: Leave the existing microUSB port on the WARBL, add a battery and a second, internal, full-size USB Host port, and just sell a tiny USB Bluetooth dongle from a random other supplier it's been verified to work with. Then be sure to always use an awkward phrase like "WARBL supports USB and also 3rd party USB Bluetooth adapters" in marketing to avoid being sued.
I actually have a big stack of still in-packaging USB Bluetooth adapters from a failed company, would be happy to send you however many you'd like (and later all of them if you want) gratis for you to play with if you'd like.
Not sure if it's compatible with whatever board you ended up using in production, but you can get tiny USB Host shields for <$4 apparently:
https://www.aliexpress.com/item/3294242 ... a9796af5-0
The only way I can think of it fitting would be to be like mounted on the other side of the air sensor, with enough room for a USB Bluetooth dongle under it before other electronics start. Given the consumer would likely only need to do this once, the procedure could be slightly annoying. IANAL, but maybe you could just offer an "optional installation fee" and then do it before sending out the instrument without running afoul of licensing issues.
FYI it seems like this workaround is not an original thought:
https://github.com/semuconsulting/Gamepad_PS4BT (scroll down and look at the picture.)
Also, a thought: If you ever get any returns that are unsalvageable in a profitable way, you should make them available for sale, I'm sure there is a market for hardware hackers who would want to try to fix & play around.
UPDATE: Another thought would be to use a board that supports both MIDI and USB Host Mode all-in-one, looks like this one does and goes for just $5, but I have no idea if its other specs are up to par:
https://cyaninfinite.com/seeeduino-xiao ... ontroller/ - although this might actually make it harder to do in an aesthetically pleasing way, as I don't think they make USB-C Bluetooth Dongles, so when used with Bluetooth you'd need both an adapter and the dongle. I can think of maybe some kind of screw-on false bottom you could put over it, but I'm guessing that'd make the mechanics of the case way more complex than they are currently.
UPDATE 2: Just realized you're doing a custom PCB, so I guess it'd make more sense to add something like
https://www.maximintegrated.com/en/prod ... 3421E.html and an internal full-sized USB port, but that'd like double to complexity of the board so I can see why you might be hesitant to go that route. When I thought you were mostly stuffing some pre-existing board in there I asked on reddit -
https://www.reddit.com/r/arduino/commen ... host_mode/? - and got this comment which may be useful in any case:
"I guess this depends on where you live (and I'm not a lawyer and have no first hand experience making a commercial wireless product), but I believe in most countries those bluetooth modules with RF shielding can be (and usually are) certified separately precisely so that people using them in products wouldn't need to go through the whole certification process again."
I'm pretty sure while searching around I've seen Bluetooth modules like this that look like they are meant to go right on someone else's PCB.
UPDATE 3: This thread looks super-useful regarding pre-certified bluetooth modules:
https://www.eevblog.com/forum/projects/ ... roduction/