It would appear that Bethesda had to hardcode in the new Crossbow tech, as even if Dawnguard is not loaded in the Creation Kit, the Crossbow weapon type appears as an option when creating weapons, and all ammo now has an optional "Bolt" flag. It would appear the only thing missing is the meshes and the condition setup; that is, the animations should still be in the files, but the conditions have not been created for them to actually play, so presumably the weapon type does nothing. The animation events for crossbows are still in the game, and could probably be rigged up under Gameplay->Animations with very little effort if one were to replicate what is in Dawnguard.
Presumably if someone could create their own meshes and rig up the proper animation conditions, then crossbows could work outside of Dawnguard.
Can anyone confirm this? Try to get a dummy crossbow working without Dawnguard, I'd like to see if it's possible to make a crossbow without it if we provide our own models and such. If it works, it'll mean great strides for modders.