On the other hand making some attempt to solving this long standing issue (of how to get many players into a game at the same time) is definitely worth spending some energy on. Maybe tweaking this approach a little more will make it really good (but I'm not sure how, yet)?
I'll throw some idea out to help with the discussion.
First, a system where people can enter in times - or a range - when they are most likely to play. This system could just be a single forum thread, and then people can edit their answers at any time.
After a month, using the information from that post, try scheduling a few games around the most common time frames that people are available.
I don't see that any kind of formal "sign-up" sheet would be necessary. As you say, Tom, sometimes people forget or something comes up, so no reason that anybody has to commit.
For instance, if we see that several people will be available from 4pm-8pm (Time zones not needed for this example
), post a schedule that a game will start at 5pm & 7pm.
So people see that, and if they can plan ahead, they are able to arrange to be in the lobby at those times. They know whoever can meet at those times will be there. Whoever shows up, plays. If there are lots of people, two or three games can be started at those times.
Another idea is to encourage people to post in the forums the times they are likely to be on. Other people will see "so-and-so" says he's often on daily at 6, or Saturdays at 3.
A third method would simply be to post in the server announcements something like "Headless server 3 will be starting a game at 5pm every day"
With all of these ideas, it has to be understood and agreed by regular participants to not start the server earlier than the times that have been posted.