There is a raspberry pi running kermit with a DigiKey usb->RS232 adapter attached.
The 486 runs CKermit. The Raspberry Pi hits a restful endpoint, grabs the midi file, sends it via kermit to a ram drive on the dos machine, and sends commands over kermit to run dos programs to play the midi file.
I used to use qbasic + curl + a packet driver on the dos machine and it did more of it's own control - but it wasn't as stable as kermit.
It's now on FreeDos, which seems to be _really_ stable. I haven't had to reboot the box at all.