This sounds awesome Jef! Personally, I am certainly not upset that you are building this on a newer version of MBSeq with more features and better support :)
My only concern is mainly a selfish one. I'm on the list to get a MB808 kit if/when it finally appears and it would be less brain work for me to have to learn 2 different interfaces for my 808/909 babies. I also thought it might be less work for you to work from the MB808 sequencer codebase since a lot of what you're doing to the MBSeqV4 code is probably similar to what has been done to the MBSeqV3 code for the MB808 project. Or that you could take design decisions from the other project where you see fit. It sounds like you're already past that point though...
One thing the MB808 sequencer does that I'm quite fond of is being able to choose arbitrary loop start/stop points for each individual part. For example, the kick might be a loop of 4 16ths (I make techno, that's all it does most of the time ;) ) while the hihat might be looping from note 5 to 12, creating some weird polyrhythm thing. Will this be possible on your sequencer? Clearly the MBSeqV4 is capable of this but will your interface make this accessible, even easy?
Thank you for all the time and work you're putting into this. I know it is a labor of love. I hope none of my comments sound demanding or too critical. And I also hope I'm not derailing the intended topic of this thread.
Cheers,
Randy