Maybe I'm just being silly here, but I'm wondering if anybody has ever used their computer for sharing files over USB. That is, the computer pretends to be a USB mass storage device. Surely, somebody must have thought of this before :-D
Yes, Apple thought of it years ago. Plug a Mac/laptop to another Mac via Firewire (and I think USB too), boot it while holding down the T key (I'm pretty sure it's T), and it boots as a slave drive.
This was a standard feature used when you upgraded hardware and wanted to migrate your data across. Not sure if it works on Intel Macs, but don't see why it wouldn't.
However, this feature also relied on the BIOS. PCs don't have this. And if you just plugged two PCs together via USB, each end would be connected to a motherboard, or a PCI host card, not an actual device.
I have never seen this done in PC land, and it would probably require hardware/BIOS changes before someone implemented this in Linux.