Make a post "I will be in the chat in three hours" Or some amount of time late.
"I am in the chat" Maybe other people can say they are there too to encourage others to come in.
"I've left the chat" Not the same as the chat is closed, but a top level post saying you are in the chat days later isn't exactly true anymore.
I do think this is a great idea, it would be good if GCC was used more often like it used to be. It will take some time for it to catch on.