These appear to be the relevant patents:
https://patents.google.com/patent/US6415439B1/en includes some aspects of the MIDI protocol in the tables; does this cover the LPC information the SWEETPEA authors couldn't figure out?
https://patents.google.com/patent/US6317714B1/en
https://patents.google.com/patent/US6281939B1/en
It doesn't look like anyone's published a reverse-engineering of the protocol.
Where does one get a 49.86mhz(??) maker-level transmitter these days?