Taking inspiration (and information about the instrument IDs) from rb4.app I just finished building a standalone midi pro adapter for XBOX One / Rock Band 4 with entirely off the shelf parts.
Mad Catz RockBand MIDI Pro Adapter MPA PS3/PS4 No sticky residue see description | eBay
Amazon.com: Xbox 360 Rock Band 3 Midi PRO-Adapter : Video Games
Roll Limitless PS4 Adapter! ~ Play Rock Band 4 With Your Electronic Drumset! - YouTube
File:PS3 Rock Band 3 MIDI Pro-Adapter.jpg - Wikipedia
Mad Catz Rock Band 3 MIDI Pro Adapter for Microsoft Xbox 360 - Black 728658024307 | eBay
PDP Rock Band 4 Legacy Adapter and Mad Catz Midi Pro Adapter for Xbox One | eBay
MIDI Rocker LX2? Yes, it's a real thing! (e-Drum adapter for Rock Band) — Byte Arts LLC
Squier Rock Band 3 MIDI Box - Sony Playstation 3 | Sweetwater
Rock Band 3 MIDI Pro Adapter for Sony Playstation 3 | Reverb
New PDP Wired Legacy Adapter & Midi Pro Adapter for Sale in Bellevue, WA - OfferUp
Amazon.com: Mad Catz Rock Band 3 MIDI PRO-ADAPTER for Wii and Wii U : Video Games
Squier Rock Band 3 MIDI Box - Nintendo Wii | Sweetwater
Convert a Wii Midi Pro Adapter To PS3/PS4 : r/Rockband
PDP Rock Band 4 Legacy Adapter and Mad Catz Midi Pro Adapter for Xbox One | eBay
Amazon.com: Rock Band 4 - Legacy Game Controller Adapter for Xbox One : Video Games
Midi Pro Adapter for xbox360, still relevant? : r/Rockband
PlayStation 3 Harmonix Rock Band MIDI Pro Adapter for PS3 PS4 PS5 RB3 | eBay
Review: Rock Band Wired Legacy Adapter – Destructoid
Will all PS3 MIDI Pro Adapters work for Rock Band 4 PS4 Pro E-Kit Drums?? : r/Rockband
Why This Rock Band accessory is selling for $1,000 | Tom's Guide
Looking to buy Xbox One MIDI Adapter : r/Rockband
Buy PlayStation 3 PlayStation 3 Rock Band 3 MIDI Pro-Adapter | eStarland.com
MadCatz Midi Pro Adapter review - YouTube
