I have built a 6 port midi thru box, and there are schematics available for midi merge and similar things available on the net.
Midi is just serial at ~32kbs. I think the problem is not the hardware which I think you can program the UARTS in the ST to do that baud rate, but the fact that cubase needs an MROS driver so that it can see the ports. I havent used any other MIDI sequencers before so I cant say about them.
