Not sure if there is something special that can be used as a spacer, I think the easiest is like Kyahx said, just make another statusbar with 0 alpha or something, then you can treat it as any other bar in the code.
Having alot of RL stuff right now I havent been able to play that much with oRA_MainTankFrames lately.
What I have come up with though when trying to find the problem with clickcasting on hostile targets using WatchDog is that it keeps coming back to that TargetByName() is used to retarget the mob which results in the wrong target being selected sometimes when you have several mobs with the same name nearby. If TargetLastEnemy() was used it would probably work but from what I can see there is some logic in WD which prevents it from using it. I'm not sure if there is a workaround for this other than modifying the WD code, perhaps cladhaire could enlighten us about that? :)