Thank you Trash80 and catskull for the help!
I tried both the version Trash80 posted, as well as catskull's fork, and neither worked off the bat. After trying swapping the usb cable, as well as using several different midi sending programs (I was using Reaper, but also tried several free keyboard programs as suggested), I still wasn't having any luck.
I eventually tried swapping the Arduino USB Type to "All Of The Above," and I've finally gotten the LEDs to respond to the midi out from Reaper. I haven't gotten it to talk properly to the GBP yet, but there's plenty of things I can try and troubleshoot on that before I give up, first being my wiring.
A couple quirks I'm running into though on the Arduino side:
1. Due to the "All of the Above" USB setting, the Teensy enumerates a ton of virtual devices, and clogs up midi selection menus due to this. A small price to pay, but I'll be trying the other USB types to see if any others work as All of the Above does.
2. I still could not get the version that Trash80 linked to to work as of yet. Currently only catskull's fork is responding for me.
3. On all versions I have tried to use, using the mode button does not loop back to 1 after 7. If I click past 7, no lights will light up from mode changes, and I have to use a clear EEPROM sketch to empty the memory. The only way I've found to avoid this is to set the number of modes in the main Arduinoboy.ino sketch to 6, in which case it goes to 6 then wraps back to 1 no problem. Is this indicative of anything specific I'm doing wrong?
4. When trying to use the USB type "MIDI", I often was getting driver crashes whenever I tried to access the Teensy midi device in any program. This doesn't appear to be happening anymore with All of the Above selected, but I thought it was worth noting.
Thanks again for all the help, I'm very new to all of this still. If anybody has any insight to the strange behavior, I'd love to hear it. In the meantime, I'm going to making sure my serial connections to the GBP are correct.