I think it's completely reasonable. It's no different from asking people at conferences, in classes, etc. And I assume the primary purpose is gauge what the can

I think it's completely reasonable. It's no different from asking people at conferences, in classes, etc. And I assume the primary purpose is gauge what the candidate actually knows about the domain, how the candidate goes about solving problems, etc., - not to actually get the perfect solution to your problem, though that would be a bonus.