Write a script to do it and you can see it in the current mIRC. This is the epitome of script usage.
By the way, the address book and automation are not dichotomous. You can script mIRC to automatically add users to the address book with random colours. In fact, one way to script this would be:
on ^*:TEXT:*:#:if (!$cnick($nick)) cnick -m1 $nick $rand(1,16)
Of course, this doesn't highlight the entire line, just the nickname, but you can use $cnick($nick).color to create a custom /echo line for the message. I'm sure you can expand on this, or you can ask in Scripts & Popups for someone to help out.