Arsil over at Mod the Sims has just answered a prayer of mine (and possibly many users of the custom careers). He’s created five vanilla rabbithole doors, much in the style of Jynx’s rabbithole rugs and doors, with all the interactions removed. And you can assign a custom career to the rabbithole. This will mean you can create a lot and have just the custom career on it.
I use the Lifeguard Career and would like to use it in the leisure centre/gym. Ordinarily, I’d have to put a Spa rabbithole rug/door/building on the lot for my sims to go to work there. But with Arsil’s mod, I’d be able to edit the Lifeguard Career and change the RabbitholeType to one of his custom rabbitholes and place that door on the lot. And therefore, I’d have only my Lifeguard sims use that rabbithole door.