I didn't make any progress after the last message I wrote, unfortunately.
I don't remember the details anymore, but based on what I wrote above — that sending 4000 bytes from the computer works ok, but sending a somewhat smaller number of bytes from the synth doesn't — I think I concluded that it's too much work for too unlikely gains.
One of the reasons for this lack of faith is that the synth in question, DSI Tetra, has some problems, or at least some unpolished characteristics, with the combo mode. That's why I think it's possible that the syxdump problems might be because of the implementation of the synth, and that there's nothing I can do on the MBNG side. After all, a big test file dump from the computer to the MBNG works ok.