For those wondering, I got back to work on this, and here you have:
https://github.com/XorTroll/Atmosphere/tree/fspusb-dev/stratosphere/fsp-usb
fsp-usb is a brand new fs-like service which will be merged into ams_mitm as soon as it is finished, which allows mounting USB drives.
GitHub's code works fine, according to my latest tests everything works fine except messing around with more than 1 drive. Anyway, anyone who wants to compile and test it (who has knowledge of sysmodules and IPC stuff) go ahead.
Things are going pretty nice