There has been a
similar discussion taking place on the VATSIM forums.
All I will say from a personal standpoint is that you must remember this is entirely a volunteer controller-base, with that said, be appreciative of what you have. Controllers can argue the same thing against pilots! Why do a large amount of pilots fail to contact us when we are online, why do they disconnect as soon as we send them a 'contact me' message, etc. And why hasn't there been something developed where pilots schedule themselves in advance so us controllers can get on line at a time to experience the most traffic possible.
A lot of controllers simply get online when they get the chance, and log off when they feel that they're getting tired, want to go spend time with family, have to make dinner, etc. We don't have controlling schedules, therefore, we usually don't know when we're going to be logging off. The controllers are just like pilots, we control when we want just like you conduct flights when you want.
Now that I've given my personal opinion/input, at Boston Virtual ARTCC we have an
ATC Timetable, which sounds like what you're looking for. It is optional on the controllers part to mark himself on this Timetable, however, we have found it to be very popular and almost all of our controllers use it. We encourage it's use so that pilots are able to know when ATC coverage will be available. On top of that, controllers use it to coordinate when shift-changes may take place.
Hope this helps for when you're flying in/around our airspace.
Regards,