Cheers! And thanks for the help guys. Although that news kind of stinks.
ESPs can refer to each other, but as soon as you open one back up in the CK the reference will be removed (so, not particularly helpful).
That may be true, but I'm not seeing that. I have an ESP file that has several new hair styles and armors in it. If I load it along with another ESP file that I'm using to modify some NPCs in, I can see the hairs and the armors fine, I can assign them to the NPCs fine... but they do not appear on the NPC in the game. Which is why I'm considering switching one of the ESPs to an ESM.