Very cool question. I haven't tried doing this before - but here's the rundown:

1) Call up some drum PART and start editing that part. I used the "Maple Custom Kit"
2) While editing this PART - use the touchscreen (TS) to navigate to the "Part Settings" -> "General" menu and change the "Part Output" to "Drum". It's the 2nd line down on the far right.
3) (TS) menu "Effect" -> "Routing" is where the magic is going to happen where you assign each drum to a different USB output. Right next to the "Routing" menu tab is the "Drum Key" option. You can scroll through each of these as changing this value will call up each individual drum.
4) Make sure "Connect" (under "Drum Key") is set to "Thru". This is crucial. Because insertion effects are incompatible with sending an individual drum key to USB outputs. You will notice if you do not set "Connect" to "Thru" then the "Drum Key Out" (far right, far bottom) will be black and set to Main L&R. Black meaning you cannot change the setting.
5) Set the "Drum Key Out" option to one of the USB outputs. Click on "USB Stereo" or "USB Mono" and you will be presented with one of a number of digital outputs.
6) Go back to #3 changing to a different "Drum Key" (meaning different piece of the drum kit) and rinse and repeat steps 4-5 until you're done assigning USB outputs to each piece of the kit you want.

From previous posts:

https://www.yamahasynth.com/forum/deleted-33#reply-19344 (See the Extra Credit section at the bottom).

Finally, Finally got a chance to try it out and it's working like a charm. Thanks!