Years ago, a classmate of mine built a rig out of a receipt printer and one of those old handheld scanners to provide an "air gap", though it never really worked (sort of an art project at the time). Might be time to revive the idea...
You could get all stuxnet and exploit the various applications (such as the components that inspect zipped content), but the transport itself is a simple file copy over a bitstream. You could do the same thing with kermit and uuencode a bit more easily.
I've heard of using serial lines/modems with the appropriate tx->rx cut, but I don't know if it would actually work for ethernet (maybe 10BaseT only?)
Half-duplex, 100mbit, ignore link. I suppose it can be done?....