Efika will work you say?
Hmmm.... Efika is small, it is low price and MorphOS just became much cheaper to license for that hardware....
That might be worth looking into, if I can find a CHEAP used Efika board.
You know, I don't mean to moan with all my complaints, though I know I'm not the first to be upset in relation to the state of USB or the matter of hardware support for said supported machines. Compatibility with software (either old or MorphOS native) that is said to be supported. After all this was to be a solution to the death of the Amiga, but it ain't much a solution if no one cares to try to make it as backwards compatible as possible, or conveniently expandable as possible at that. Especially when it is starting to look like MorphOS version 4 is going to kill backwards compatibility as we know it all together. Woohoo! Bright days ahead.
So ya, given that we just jumped from 2.7 to 3.1 and said supported hardware still isn't fully supported (no wireless in MacMini, USB is still crippled compared to competing OS') and backwards, compatibility still hasn't been fully fleshed out, and the fact that the future may look like the death to all that... I do tend to moan a little.
All that senseless crap aside, they got my money now, and half of me does not feel raped. Half of me really likes what MorphOS is (for what it is), and I would find it hard to let go of. As I have said time and time again, unlike AROS (in its current state) it runs quite good under the hood. The OS specific features themselves have been fleshed out quite well and have created the best OS experience I've had since classic Amiga OS with Magellan II, and BeOS/Haiku!
Though, OS strengths aside, MorphOS' road map is starting to sound like AROS. To dedicate all efforts in getting games to run or run better, and having something to browse the web with. For us creative guys this isn't enough.
Anyhow the last I checked the Browser is pretty much a one man job. Seems to me (wireless aside) that the network stack is pretty damn good on MorphOS, so how could that require so much of the teams time and energy?
3D seems alright to me on MorphOS. Sure we don't have support for super high end cards and the ability to play DOOM 3 and what have you, but again, I don't care atm. I'm not pressed to play any more games, and besides, AROS already provides in that area.
What we need an a next Gen Amiga OS that is willing to continue to work out backwards compatibility issues (as a main priority), has Wireless support (AROS actually does have that but they have no wireless manager, irony of ironies), and we need USB that ain't stuck in the far past. We need cheap means to expand our hardware of choice and should not be forced to buy a different machine or OS for each task.
I would look into OS 4 as a solution as it seems to have support for some of these things, but alas I hear it is not quite as good under the hood, nor is it cheap to run.
BTW, I can sample on my Atari 8-bit (maybe not my C64), I do have MIDI on my C64, and I can do both on a 500, if I had one, but since I have a 600's and 1200's, I use them instead.
I remember (this was about 10 years ago) when MorphOS didnt have 3D acceleration for Radeon cards there was somebody yelling how MorphOS could be taken seriously without 3D... to some user other things are more important that others.
Neither my Amiga 500 or Commodore 64 support sound sampling or MIDI... 
as far as USB isochronous transfer support is concerned it is not something that very often. Anyway, I have no idea if isochronous transfer will be supported. It would be nice to have so Poseidon support would be complete but it is not my job.
Anyway, at least you have raised the issue. If nobody ever ask then nobody care...
Edit
For Maestrix emulation have you tried this:
http://aminet.net/package/util/libs/TheMaestrix
I dont have OctaMED SS so I cant try it out (and you probably already tried it?). If it doesnt work I dont know if there is something else blocking OctaMED SS but again... I never had that program.
To use MIDI you would need a computer with old style serial port (Pegasos 1/2 or Efika ?), it certainly is supported in MorphOS but to my understanding MIDI via USB would require isochronous support... so it comes back to USB then.
