Turns out it is possible, but you have to dig pretty deeply into Capybara’s guts to make it work. If you’re using Cucumber, just drop this into your support directory:
Then use it by calling it within a step definition with whatever fields you want enabled.
It should also adapt easily to non-Cucumber environments; just make it accessible and call it alongside the code that attempts to set the fields.
Hopefully this saves someone else some time; I’ll never get that half-hour back myself!