You don't actually need much, for a form I used to get spam in I just added a "write 42 here" so anyone who actually cares to read would be able to fill it. spam fell to 0.
(for a site with a slightly higher profile this wouldn't be enough, but for a minor corner of the internet with no ill intent actually aimed at it that turned out to be enough to block the fuzzing "fill all the forms" spam)
As contrasting experience, I did that (a simple math problem) on our contact form and it did NOT drop spam to zero; our spammers were too smart for that. Even an actual reCAPTCHA didn't completely eliminate it (although it mostly did, enough that it's fine for us).
Similarly an empty input field that is css'd to be outside the viewport is often filled by spambots but not humans. But I like the edge case UX of your idea more.
Just watch out that Chrome’s autofill doesn’t fill it in. Cost us a huge chunk of new signups until we found out. Chrome ignores autofill directives under some circumstances.
It's also visible for users with CSS overrides and/or other browser inpairments. The more I think about it the more strongly I prefer the "type 42" explicit input field.
(for a site with a slightly higher profile this wouldn't be enough, but for a minor corner of the internet with no ill intent actually aimed at it that turned out to be enough to block the fuzzing "fill all the forms" spam)