Just wanted to share my experience with everyone, so hopefully nobody repeats it.
Info for rooting/unlocking/flashing recovery for the ATT HOX+ is quite disorganized at this point. Kinda went in blind, using experience from multiple other phones.
Flashed TWRP after unlocking bootloader. Could not mount SD (no option, was using the version on TWRP's website before I'd found this thread). Thought perhaps WIPING DATA might fix this. F*#@ me. Softbrick.
Found this thread. Realized I'm an idiot (though in fairness, WHAT THE HELL?! WIPE DATA=softbrick?!?! Never seen that one before).
Flashed new TWRP in this thread. Mount SD button shows up. Try to mount. No good, device not showing up on PC (Win 7 x64). Try to ADB sideload. Also no good (error: device not found). Maybe bad drivers? Searched around, updated binaries, etc. Still couldn't push anything to SD.
Give up. Return phone. New phone. Keeping stock until there's more development =/
Also, FWIW, all this was only an attempt to fix the bluetooth on the phone, since bluetooth audio quality is substantially worse than on my original HOX.