We are currently planning some work on a similar feature, custom validations: Custom validation - #9 by mat_jack1
While it’s not exactly the same, it could technically still fulfill this need (with a little more effort — you’d have to write that logic in code, rather than in a nice flow GUI). Do you think that’s similar enough that it could be helpful? If so, it might be worth mentioning this use case in that thread too, so the devs know to consider this need as well.
If you think they’re different enough, no prob. This feature request will remain open regardless. (It’s just that the other one might be released sooner than this one)