The more I learn about the YDS the more I'm starting to appreciate how unique it is from EWIs and Aerophones, even the WX5. It really does seem to have a lot of different intent as a standalone tool. I heard back from Yamaha and was surprised with a really in-depth phone call, so props to their customer support and outreach and everything! There's still going to be a bit of waiting for specific information as my specific question had to be forwarded on, but in the interim, I was able to get a glimpse of a midi implementation chart. Here's a quick summary for anyone that's curious:
The YDS-150 cannot receive midi signals from other instruments, unsurprisingly. Midi information is sent on channel 1 and that’s the only channel available. It uses OMNI OFF, POLY mode which just means, again, it only sends one channel. POLY mode is interesting, though. Note range is 0-127 with no true voice aka the transposition of notes on the outer limits. Interestingly enough, the entire range of velocity IS sent as well as notes’ off velocity. That means it's not like I was assuming. NOTE ON isn't "9nh, v=127" or "9nh, v=1-127 -> 127." After touch isn’t sent, pitch bend is (along with modulation on CC#1, portamento on either CC#5 or 65, expression on CC#11, and binary information on some other channels). Program Change isn’t sent meaning voices aren’t sent. The YDS sends onboard voices through Aux/Phone, though, so that makes sense. Nothing from Common, System Real-Time, and Aux Messages are sent; that's things like click track, real-time intonation, timestamps, etc. It says System Exclusive messages are sent and received without a note, so I'm guessing there's nothing special there.
It's nice to know that the full range of velocity IS sent. I'll have to play around some more with how the YDS interacts with Ableton.
As a separate matter, though, I'm starting to wonder if there's a possibility of creating a custom mouthpiece that can interact with the pitch bend joystick. It's pretty easy to get into the back of the YDS, apparently, if you Allen wrench off the left-hand upper stack of keys and the right-hand lower stack then take the screws off the rest of the plate. Maybe something like a
flex sensor or an
fsr could be wired into the signal then hidden in a homemade mouthpiece. No idea where you'd keep the Arduino, though...