The project is a difficult one.
The reasons are twofold,
a) USB support on DOS is limited.
There are minimum device drivers, like for HID (keyboard and mouse) and for Mass Storage (HDD, Pen drive, CD/DVD). But not so much for other USB classes.
Then, you have to write your custom driver upon a low-level device driver, such as DOSUSB.
b) USB MIDI products use custom drivers, developed by the manufacturers
The major manufacturers (Roland, Yamaha, Korg etc) of MIDI instruments develop their own custom device drivers, which (slightly) deviate from the standard USB Audio MIDI class. You have to decipher the protocol first, for each device. And then, implement it to your DOS driver.