When we devise solutions, sometimes we have to enumerate them first, before we can think about them in sophisticated ways.
Programmable Read Only Memory is supplemented by Erasable Programmable Read Only Memory, and Electrically Erasable Programmable Read Only Memory. The one-way function of the Programmable ROM (usually ensured by ultra-violet erase, with a filter film covering,) would render it similar to a CD-ROM or a write-blocked SD Card.
PROM would differ from RFID in being subject to user initialization, with the necessary equipment, but thereafter becoming tamper-evident. This improves its appeal as an alternative to USB, for a dongle. An SD Card, with the write-block enabled, would serve a similar purpose.
Recent Snowden leaks assert NSA can tamper the USB channel. One would normally face this degree of scrutiny only if the information in question were of interest to a nation-state. Disreputable nation states subsidize industrial espionage, but the silent majority do not.
PROM’s have limited read bandwidth, so read-intensive operations would slow processing, which is usually not desirable, but could possibly be turned to advantage if creatively employed.
Theorizing infinite time, one could select a position on a CPU bus, to introduce an encrypted CPU core. This might differ from encrypted processing, or assist implementation.
A removable PROM could serve as a password-key which could be updated by replacement. These would not hot-swap, and current PROM’s are not manufactured to accommodate constant cycles of removal and re-installation.
Alternatively, we can theorize a hardware USB key encryptor, which implements, for example Truecrypt, with each hash and/or encryption algorithm on a separate (P)ROM, and designated by selector switches. Password entry might be accomplished with yet another PROM, or dip switches, or rotors a la the Enigma machine.
This scheme would obviate the requirement for the USB key end-user to know the password, and the USB drive would be secure in transit.