OK, well I've been experimenting today and did get it working--for the most part. First of all, I think softmpu is trying to use an unsupported baudrate or something, since after loading softmpu in autoexec.bat, I have to manually set the mode (using the 'mode' command) to a specific rate. For some reason, setting the mode in autoexec.bat does not seem to work, whether the line is before or after the one that loads softmpu. Not sure what that's about. Also, the mode command does not seem to support any rate higher than 19200, as opposed to windows, which goes much higher. Why is that? I also tried setting it to something slower, but at 9600, the midi coming from the linux machine sounds drunk.
Here's the setup:
dos machine: intel pd440fx motherboard with a pentium 2 266 and 128MB or ram, onboard opl3-sax and running softmpu
linux machine: ubuntu 18.04 with JACK audio server and mod-ttymidi and fluidsynth/arachno 1.0 soundfont as JACK clients.
They are connected via null modem cable at DOS's apparent maximum baudrate of 19200 (and believe me, you need that max speed)
Only question I have is, is softmpu attempting to set the mode for the serial port at all, or does it expect users to do that manually. It is not documented. As I mentioned earlier in this thread, the baudrate for "serial midi" is 38400, so perhaps softmpu is trying to set the serial port to that. But that wouldn't really make sense, since the max supported by dos is 19200. I'm confused.
Lastly, the sound comes through just fine, but it doesn't sound right. This may have to do with the arachno sound font, but I did try to use one of the GM sound fonts that came with fluidsynth, and that did not sound much better. I'm running monkey island 2 with the 'monkey2 r' command, which just says roland, but nothing more specific than that.
Anyway, that's what I got so far. Any more information would be helpful.