The only one thing on WotLK required to enable new race/class combination on !client! side is adding a new row into CharBaseInfo.dbc. I wouldn't expect it to be too different on Cata.
Auuuuhm. First of all, CharBaseInfo.:https://wowdev.wiki/DB/CharBaseInfoSimply allows which combinations of races and classes are allowed. Which means that you simply do want to add a new row there - with UD race ID (don't remember which one is that now) and class ID 2.And about editor... MyDBCEditor can't open that DBC, but for example Taliis can. And while I absolutely hate Taliis and discourage everyone from ever using it for anything, this is probably the only exception I use Taliis for. And I'd be surprised if for example this new DBC editor:viewtopic.php?f=59&t=10692wasn't able to open and edit that DBC.