It's possible, even feasible, and I'll explain how to do so further down. I'd like first suggest that in the absence of a guide mode loop on your belay tube, your likely going to be better of using another system like the auto-locking Italian hitch, than using your ATC XP or similar.
Read here for more information on locking Italian Hitch: http://www.climbing.com/skill/munter-magic/
That said, you could still do it by extending your belay station. Here's how:
- Once you've finished constructing an anchor, clip your followers
rope(s) through your master point.
- Lower yourself a certain amount to a comfortable belay spot, say 10 or more
feet to a ledge / edge, or just hanging if needed.
- Belay from your harness like you normally would on the ground, using
the strand(s) of rope that are now redirected through the anchor
Note: If there is a weight differential, you may want to anchor yourself down using a tree, slcd or other piece of gear, just like you would want to if belay at the ground.
I have employed this system myself in certain scenarios, mostly topping out on blocky terrain, where you are slightly less worried about your second falling, but are concerned about lack of visibility or communications. As the probability of falling increases though, so should your inclination to belay directly from an anchor. > It simplifies things a lot should you have to do any self-rescue for your group.
Update: There are several ways to make a belay locking and I'd encourage you to read the following book if the topic is of interest to you: http://www.mountaineersbooks.org/Climbing-Self-Rescue-P262.aspx. For the sake of inclusion though, here is one method that most people should already know. Any type of grabbing hitch tied to the brake side of the rope, mostly commonly used when belaying yourself (rappelling), but you could can certainly use it to belay somebody else also.