Is it possible to mount a disk brake hub to a rim-brake rim? Suppose I have a disk-compatible hub and frame, but want build a wheel based around a very specific touring rim, which is labelled as being for rim brakes.

My understanding is that you cannot attach a rim brake to a disk-compatible rim, since the sidewalls are not designed to withstand the stresses. However, it is not obvious to me whether there would be any problem attaching a disk brake to a wheel with a non-disk rim (assuming that it's been built around a disk-compatible hub, and is attached to a disk-compatible frame). 

Is there some problem with this configuration? The obvious drawback would be carrying around the extra weight of a reinforced rim, but that doesn't seem to prevent the wheel from actually functioning.
There's no problem mounting a disc-compatible hub to rim-brake compatible rim. Just make sure they've got the same number of spokes and you're good to go.

You're absolutely right about the opposite though, disc brake rims generally do not have a braking surface on them, the rim profile is round there, so it wouldn't work. And if you did try to brake on it, it a) wouldn't work well and b) would probably cause excessive rim wear.

What you could do, if you want to be flexible and switch back and forth is to get a dual-duty rim that supports both, such as the WTB Dual Duty FR rims (probably a bit beefy, but you get the idea).