I called Pioneer support and they're clueless beyond what's written down for them. I still don't know if the DEH-PRS80 can support up to 2TB FAT32 partitions from its USB port.
2TB is the theoretical limit of FAT32, though people usually use Windows NTFS or a Linux EXT partition schema when they go that big. When it comes to car audio, manufacturers only support up to FAT32, so I can't go with a regular PC file allocation schema for large drives.
I posted on their Facebook wall, so who knows, I might get an answer there where support couldn't help. Otherwise contacting Pioneer's Harry Kroll directly is my last resort. If I never get an answer, I will just try it myself after purchasing the DEH-80PRS and return it if it can't support the full FAT32 spec.