Found this on the castersrealm boards. If you put the lines:
[Main]
UISkin=*skinname*
in your Charname_1.ini file, it will load some of the graphics from the named custom folder.
I played with it a little and it looks like you have to put it in all your char's ini files or else it will use the graphics from the previous logged-in char's skin. Also it didn't load my other window background images.
Probably needs some more playing with. I didn't try zoning with any chars after logging in, for instance. Not sure if that would change anything.
EDIT: Zoning doesn't do anything. I tried putting that line in every char.ini file and each char loads up with the same spellbook graphic as the first char I logged in.
No new ideas.