I like this but have a slightly different idea.
Build this into the event scheduling system.
The filtering options you mention are pretty much on the Event Schedule window anyway with the exception of LEVEL and the filter for Mains/Alts/Both (I like this filter because one reason for a non official run might be to gear up some alts that people have fun playing). The other benefit in doing it through the Event Schedule options are that it has a filter for Minimum Rank on it also. A standard user organized run might not care about minimum rank but it would be there.
It might even be possible to add the level filter to the Event Category definition. Most systems are going to have categories already defined. The ability to over-ride them on the Event Schedule window should be included.
You should also add a field for the expected End Date and Time to the event Schedule Window so that filter matches can correctly find the number of players available in the time block requested on the schedule.
Another advantage of using this filter matching in the Event Schedule system.
Sign up requirements. Maximum Numbers and filters on Characters for rolls. The end display after choosing the filters and querying the availability could show you the numbers of characters broken down by class and roll according to how you want your signups run.
At the bottom of the Event Schedule you could change the options slightly. Have a big button that says "Check Availability" and clicking this would generate a popup window with the numbers that matched from filters decided by the specific schedule request and options chosen. Then below that you would have "Cancel" to close the scheduler and return to the previous screen and "Schedule This Run" to commit the filter options selected. You could possibly even have the Schedule and Cancel options appear on the popup window that shows the matches so a user looks at it and decides that there are enough people who's availability matched and they want so submit it or if they don't think it matches they hit cancel and the window closes returning them to the schedule page they were originally on.
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Six Demon BagJack Burton: Hey, what more can a guy ask for?
Egg Shen: Oh, a six-demon bag!
Jack Burton: Terrific, a six-demon bag. Sensational. What's in it, Egg?
Egg Shen: Wind, fire, all that kind of thing!