Somebody asked this a while back, and I don't think they got an answer. I'm curious too. Don't people install some sort of material that acts as a bridge across the angle (like a piece of wood or something) and attach the fixture to that?

If the light is at the ridge, we build a box of plywood for the electrician to mount his junction box, then cover with drywall (or ceiling material) and finish to match. On wood ceilings, we use a large piece of wood and create a solid block to mount the junction box. We always block the framing around the wire before any finish material goes up...we never know when a ceiling fan will replace a simple light fixture. On a sloping ceiling mount we simply use a cathedral ceiling canopy and mounting system. Either a solid down rod with a chain link at the top or chain instead of the solid down rod.

Any type of fixture would work if it has a canopy and chain or rod that can hang on an angle. You could also use track, this could have directional heads for certain areas and a pendant as well. Many suppliers also provide additional rods or chain for very high ceilings. As long as a fixture has the ability to be hung on an angle you should be ok. If this room also needs ventilation, you could also have a fan with light kit, again make sure it can be used with a rod so that it can be hung on an angle. Hope that helps!