Alas, I come to complain about MorphOS

[zylesea]

but to my understanding MIDI via USB would require isochronous support... so it comes back to USB then.

That's not the case. usb midi uses _bulk_ transfers. But requires a working usb streaming class. The ammount of data is rather small, hence no isochronous transfer is required (isochronous is only indicated when precise timing AND high throughput come together). For midi the high throughput is not the case. usb midi is well defined in the usb specs - compare http://www.usb.org/developers/devclass_docs/midi10.pdf

I could imagine not too much effort is needed to make midi work well on MorphOS.

[zylesea]

So set up a **** 1ghz or 2ghz pc with line in to record whatever you need. Transfer recordings via usb thumb drives, problem solved, but I agree with you...

Many amiga musicians want to move to morphos or aros or aos4. There should be easier solutions for something so simple as recording audio or creating samples...

That's similar to the way I get analogue audio data to my MorphOS machines, but I use a digital audio recorder (iRiver IHP-120, old but pretty brilliant sound) connected by usb (msd class) for that purpose. It's rather easy to do.
That said most MorphOS capable machines come with a line in connector (never used it though), the mini is special in that regard (once suported the ibooks will share that limitation).