Remember everyone that ALL midi interfaces on any of the ROS products from Rhapsody right through to Sibleus 7 , where ALL implemented as Relocatable Modules.
Which module you loaded in depended on which driver you wanted to use, and the original Acorn MIDI interface used SWI numbers in it's relocatable module that where somewhere in the 0x404XX range.
Chris's acorns (http://chrisacorns.computinghistory.org.uk/
) I believe has a PDF scan of the original MIDI programmers guide.
All you would need to do would be write a Relocatable Module, that made a public interface available that exposed a compatible set of SWI vectors available in the same number range.
Midi compatible programs should then in theory find these SWI vectors and start sending MIDI Data to them.
I have some C source code kicking around somewhere for a serial port midi driver I started writing for my A5000 many years ago, but never got finished. At the time I had a Yamaha MU10 XG that had a MIDI Serial interface on it, which I wanted to use with RiscOS apps. So I started to write my own MIDI driver. I'm sure anyone with enough passion and knowledge would be able to build on it and bring it up to date, changing it to work with other hardware in the R-Pi version.
still crazy (Even since the days of my BBC Model B) BEST and only way to be
IM: @shawty_ds on twitter
if you remember the Acorn and BBC days then I was "!Shawty! of DSPD" (Author of the BBC B Sound Tracker suite, and the Dreamscape demo)