Hi Stewart, and welcome to the forum!
I'm afraid the Beocord 5500 is the only piece in addition to the Beomaster that has a chance of working with your MCP. It may be though that the Datalink codes were changed between 5000 & 5500, so I can't be sure about that either. But it's easy to check - just plug it in to the Beomaster and try! You will need to have all seven pins remaining in the plug of the Beocord though, the two Datalink pins (at the ends of the "arc") are removable so they might be missing.
Do you by "RS232 interface" mean the 8-pin DIN connector, not marked as AUX, AMPL, or any kind of A/V connecction? You can connect a serial printer to it and print Teletext pages! I dunno if that actually is an option, or present in all LX2800's...