Old-school solution:
dd if=ubuntu-{version}.iso of=/dev/{device}
After several failures and various blind alleys around disabling automounting, I discovered this comment which happens to describe exactly my situation (creating a 64-bit installer on a 32-bit platform):
Right, this explodes because the architecture you're running on doesn't allow starting executables of the architecture you're trying to burn on the USB disk.
Everything is obvious in retrospect...