But it would be even better if mIRC watched for the Nickserv "identified" message

I try to avoid solutions that depend on wording. Not only do different networks use different wording but they can change the wording and format from ircd version to version. In addition, this assumes that the wording is in English, which it may not be. Unfortunately, I have been forced to check for specific phrases in English to parse some numerics due to different ircds using the same numerics for different features. It's not a pretty solution. In this case, looking for a message from nickserv to confirm that a user has been identified is not ideal. Such a reply should really have been a numeric to make it clear to the client what has happened.

mIRC rejoins channels, and triggers the perform section, as well as initiating other features, only after it has received the MOTD. I could make mIRC wait for three seconds after the MOTD before doing this to get around the nickserv limitation but this would affect all users and all connections.

It would not be possible to make mIRC do this only if the connection has Nickserv set. Nickserv can identify your connection from an SSL certificate, two SASL formats, two Nickserv formats, and a password format specific to that server.