The Korg iMS20 MIDI spec says that CC 111 does STOP/START - I don't know if this is a toggle or whether value 00 stops and value 127 starts, but let's assume the Korg is expecting B0 6F 7F (CC 111, value 127) as the start command (and value 0 as stop). Thus your start rule would be:
FA = B0 6F 7F
However, there is a bug in the Stream Byter (fixed and I will push out a bug fix release soon) which needs to be worked around.
Here is the ruleset to do that and workaround the Stream Byter bug. For the bug we have to map to FA to F0 before doing the mapping:
# convert MIDI START to iMS20 CC # including SB bug workaround FA = X0 F0 = B0 6F 7F
NB. this breaks sysex messages!
Now, I imagine the next thing you might want to do is map FC (MIDI STOP) to CC111 value zero to stop the sequencer.
Here is the best ruleset that does start and stop (and doesn't break sysex):
# convert MIDI START/STOP to iMS20 CC # including better SB bug workaround F0 = B0 7F 40 # preserve sysex FA = X0 F0 = B0 6F 7F FC = F0 F0 = B0 6F 00 B0 7F 40 = F0 # restore sysex
I tried all three of your suggested rules (start rule, workaround and best), and experimented for some time with small changes using guesswork but I've had to accept that nothing I have tried will actually start the sequencer on the iMS-20 (it's the latest version).
Any further clues or suggestions would be very gratefully received! Would waiting for the bugfix update that you mentioned possibly help?
Since you have MidiVision, I would use it to make sure the start/stop messages are indeed being remapped to the correct CC message. With MidiVision already running:
First ensure your Stream Byter setup for the above is on the input port (on the left in MidiBridge) of whatever is sending start/stop. For now, just use the middle ruleset I posted originally to keep things simple.
Connect the above input to MidiVision on right as well as the Korg, so the event stream is being sent to both places.
Send the FA (start) and see if it is translated to CC 111 according to MidiVision. If not, then there is something amiss, in which case email me and I will give you instructions on how to turn on developer trace so I can see what is going on. It is likely to be something simple.
I tried that just now. When StreamByter is enabled I get an extra orange message in MidiVision that is not there when it's disabled, immediately after the MIDI Start message arrives. I'm listening on the same MIDI stream as the iMS-20.