It looks to me like the bolt quiver is located on the back left hip, if I remember right. I'm thinking this means any mod that touches the skeleton.nif (and there are several IIRC) will need to be modified. I can think of at least one mod I love that changes skeleton.nif and the author seems to have disappeared. Maybe someone more experienced with Nifskope can make me feel better and say yes, Bethesda had a node for this quiver all along in anticipation that they would one day add crossbows.... But I don't think so. At least I've never seen it while my clumsy hands went tinkering and breaking things in Nifskope. I think this means that if we get the expansion we'll either have to give up certain mods or hope the mod author's update them. It also means authors who own Dawnguard will need to make Dawnguard and non-Dawnguard versions. I know many, many mods were released that accommodated people who had Shivering Isles and people who didn't, but of course that's their decision, not ours.

Any modders out there who have played Dawnguard? Does it looks like Bethesda changed any other core files, assets, settings, etc? I have this terrible paranoia that they've changed the leveled lists, though I doubt it. I always get paranoid around patch/expansion/DLC time.