I have and use two Atari ST emulators for the Mac, NoSTalgia and PowerST.
http://users.skynet.be/sky39147/
Both are capable of processing MIDI through either QuickTime or OMS.
http://www.opcode.com/products/oms/
I think I'm pretty much set to try this except for one thing - how do I get MIDI into my iMac.
I've seen a few products but all do far more than I need. I'm not looking for anything fancy. Just a basic interface and some driver software. I still use my 12 year old Atari running Cubase 3.x for music sequencing/recording and was curious how that setup might work out on iMac.