index on custom headers for fast ipurge

[ram]

I am setting up a cyrus imap server with 40k-80k mailboxes

I have a particular requirement that some particular messages  should be
deleted from all mailboxes.These  are the usual yada-yada system
notifications no one reads

For this I plan to add custom headers in al these notifications and run
ipurge for all mails with that header 

Is this possible ? 

Alternatvely I could use an ImapClient script to logon to the mailboxes
and delete the mails. But I wont have passwords for all mailboxes ?

Thanks
Ram