I have done this many times at many sites Russ. Not brain surgery 🙂
If you do not want to mess with resend procs then it is simple enough to write a simple proc which goes in front. Simply look at the MSH.10 (I think that is correct) for the Message ID. Save it off in a global, or more preferable, in a namespace variable. Do the same with a count. If the account exceeds your desire, send the mail. I use tclMail but you can use what you want.
As for doing it in alerts you will be able to do that using the Tcl defined alert in 5.7.
