As an Amiga musician, I find it a bit frustrating that there is so little information about MIDI compatibility from the makers of the new Amiga clones (A-Clone, MiniMig, Natami, Sam440). Of course, most of these people are not musicians....but it would be nice to know if and what MIDI interfaces will work with these machines? The old Classic interfaces? New PC/Mac USB interfaces?
I cannot speak for the others, but Natami has a serial port which is directly bangable by the Amiga custom chipset registers. So Amiga serial peripherals, including MIDI interfaces, works just like on a classic no matter what software is used. (You will need a 25<>9pin D-Sub converter though).
External MIDI is one of my personal intended usage areas for my own Natami system, and it has actually been discussed quite a lot on the Natami boards. And we actually have quite a lot of musicians and people interested in audio tech on the team - so do not worry about that.

USB to MIDI dongle support is a matter of supporting it in the USB stack, so Natami is in the same boat as other classic Amigas here. Poseidon USB has simplemidi.class and camdusbmidi.class. But I think most serious musicians would prefer the no-latency glory of the Amiga serial port to a USB dongle. There are even musicians going back to sequencing on their old Atari STs and Amigas from their modern Macs just because of the total responsiveness of the MIDI interfaces as compared to USB solutions.
I will recieve my own Natami board shortly, and I will play around quite a lot with my HW synths. Just need to get hold of an Amiga MIDI interface again, currently short on that.