Generally these are "unseen" features such as adding multicore support or adding support for a power saving feature on your graphics card.
> How are they going to try things out if your OS doesn't support any of the techniques they're interested in (i.e. novel ways of getting at hardware, novel ways of doing virtual memory, etc)
They are interested in a lot more than that. Check the 3L references page for a few examples. [0] Plus you can do many of those things with 3L.
> You realize that this sort of thing is a huge area of current research, right? Because it's a really, really hard problem?
Yes. But it is certainly possible and has already been done to some degree. Genera, for example, did it and a number of other real-world and research OSes have done it. Of course the real challenge is making it as real-time as possible but getting it most of the way there isn't actually that difficult.