The issue I highlighted was the fact that if you want to build a USB master you're pretty much constrained to having a full-blown OS etc. in there.
Not really, you can implement USB on a microcontroller with bare metal code. A RTOS is useful to handle other tasks like scanning the keyboard matrix.
OK, I've now got a small box here that does implement a USB host, but a review
http://sandsoftwaresound.net/review-midiplus-miniengine-usb/ does comment that it's an unusual capability.
I've been keeping my eyes open for something like this for a while, since MIDI is useful on servers for monitoring purposes. It's getting increasingly unusual on expansion cards (and those cards that are available tend to have dodgy software support) and software emulators tend to have access rights issues etc. which I'm sure will be fixed RSN.
Input options are either MIDI (via an adapter) or USB host, it can't be plugged directly into a PC despite having a micro-USB socket which turns out to be power only. It works out of the box with a USB->MIDI adapter, and also connected directly to the USB port on a keyboard (i.e. box acts as host, keyboard as slave).
Internally, it's got a "Dream" SAM2635, an STM32F105, and an MXIC MX29LV640-EBTI (if my scrawled notes can be believed). I've not attempted more detailed reverse engineering, since I've bought this with a fairly urgent need rather than as a toy.
MarkMLl