I forgot to add. There is no reason User X has to run his remailer immediately. His software could simply commit to running a remailer for 1 hour at some specified future date < some threshold. Any messages sent to him for remailing would be queued until that time. Therefore, all your technique would tell you is that the user remailed a message sometime between date X and date Y. if Y-X > few days to week or two, the intelligence gathered on User X is miniscule. Traffic analysis would detect User X using the remailer network anyway.
When I've thought about this, it's been from the p.o.v. of message senders being able to earn prepaid service tokens (not unlike digital cash) for offering their machine as a remailer for a set period of time or number of message or total bandwidth or whatever. This activity could be completely asynchronous to any origination of messages, and, in fact, a regular habit of accumulating tokens like this would make for excellent cover traffic.