One thing you should definitely be careful of (not sure if you do this or not) is requiring adherence to a process that demonstrably doesn't work for a sufficiently large percentage of the population. As another poster pointed out, they select against people who don't like to pair program, which cuts out a large percentage of the (good) developer population, thus fostering monoculture.
Agreed. The "open dev time" idea is attractive, and I can imagine developers getting excited about it, (or at least developers who enjoy doing side projects). But it doesn't seem to address kstenerud's point.
In our interview process, we want developers who like writing software, and want to improve. Asking about side projects, software topics they're researching on their own, languages they're playing with, seems to be a useful thing to focus on. Or at least, I tend to like candidates with good answers to those questions. That's a monoculture I think I'd actually like.