Part of the reason that you arn't seeing the system being used much is that all the default events are being posted up, empty for all the times and dungeons they're available. That and the fact that there really arn't that many groups running them. The event that I see getting the most usage is the Crypts/Tunnels that Tearney and Mort have been running.
I think the issue for some of our groups, particularly the Lost Vale and ToVL runs that I've lead, is that we have a set or nearly set group already and don't want to risk any hurt feelings.
For example, the LV run that we've been running on Fridays is pretty well set: I have a full group and a couple alternates who are willing and ready to make the run. I know the guys even ran some of it last week while Hell and I were watching The Expendables and eating ribs for our birthday. This is a fantastic setup for everyone involved, and we've been trying to rotate in new folks as others get geared.
The problem that arises with using the signups is that if one of us signs up to lead the event a few days beforehand, but not everyone in the set group gets signed up quick, other people not in the group can throw in for it. Then, our group either has to tell our normal member they have to bow out because of the signups, or we have to tell the person who used the signup system that we can't take them. The potential for this to cause bickering between tuatha members seems awfully high, so we simply don't use the signups. It's just easier that way.
I'm not really sure what a good solution to this is. I don't think it would be a good idea to have a "Closed Group" option, as that would likely be looked at as a sign of elitism, though that's exactly what we're doing. What you mentioned you're doing with the lists of people interested in and equipped for running different dungeons is a good idea, and is something that I think even premade/closed groups would use to find substitutes.
Even now as I look at the events I see that Docroc is signed up to lead Lost Vale tonight, and Sigramz is attending. Last night I'd asked Doc on vent if he was interested in running with us and he seemed fairly enthusiastic about it. What am I (or Doc, or anyone) supposed to do? I pretty well have the group set up otherwise, and I would have to tell a regular to sit out if we take Sigramz. Or the other way around. Either way, it puts one or more people in a tight spot. There are alternatives, and some which have good possible outcomes. We could do 2 groups instead of one, I could look for a new second healer, and Doc could poke around to put the rest of his group together. Something like that 'Available Dungeon Runners' list would be fantastic in this situation, but right now it would be a crapshoot for us to put together 2 groups.
Anyhow, I'm kinda frustrated with the system right now. Either we use it, and tough out the problems that it can create, or we don't use it and give the appearance that no one is running dungeons. Oh well, enough of my crying.