I think it sounds like a really sweet idea!
Suggestions for how such a calendar could work.
1. You select a date and a time for when you wanna host the game.
2. You type in the name of the game you wanna play
3. You type in your servers IP, or if it's the case of public servers, the name of the server (if you're running the server yourself I think it would be good to also post what Internet connection you have and the number of players that are allowed on the server)
When looking for games the calendar should automagically compensate for what timezone you're in. You should be able to sign up in an easy way.
I guess that's all I think is really needed (for starters atleast), perhaps the ability to leave comments, like saying you will have to leave after an hour or similiar would be nice.
The calendar wouldn't necessarely have to be here on amiga.org, but it would be nice because lots of people would see it and hopefully use it if it was.