IsoCON doesn't utilize a server model, just p2p connections, so it's not able to do async play currently. v0.8 (coming soon) will enable sessions to persist to a file, so it's theoretically possible if everyone connects at least once to get the token graphics synced. However it would involve passing around the session file and updating it, which is cumbersome and presents some other issues.
I'll keep async play options in mind for the future, but at the moment it isn't supported.