USB MIDI Host on a microcontroller
I'd recommend taking a look at Vinculum chip by FTDI. It is specifically designed as external host hardware running USB stack so that your MCU doesn't have to do it.

Another, somewhat harder to use, solution is SL811HS from Cypress.

Both are available on convenient breakout boards, like this one for Vinculum or this for Cypress.


  I can't find additional info regarding the USB MIDI Host stack on
  Microchip's website.


What info are you looking for? If you buy that HobbyTronic board all you have to do is download and install their free MIDI host software. Then the board will convert between USB MIDI data and Serial TTL packets, which you can read/write using any MCU of your choice.

UPDATE:

Just out of curiosity, why don't you use some of the off shelf converters with USB host functionality, like Cerebel, Kenton or iConnectMIDI4+?
They all can convert USB MIDI to normal serial MIDI, easily connectable to any MCU. Admittedly, they could be pricey, but if you count time you need to make hardware, write software and cost of parts, they quickly begin to look rather on a cheap side.