As Doqnach pointed out, highlights may well be triggered by e.g. regular expressions or "changing" identifiers like $me. We'd need:

• Either an event-bound identifier to track all occuring highlights with scripted means (in respective events, suggested more than once, and I'd prefer this one. Have e.g. a look here . )
mIRC "knows" when a highlight occurs, but atm you have no access to this "is a highlight/is not a highlight" assertion. You have to check on your own, which produces overhead - and is a pain, at least.

• And/or some kind of new, prefab "bookmarking" feature (for the more casual user, thinking of the "track urls" window. Maybe with some simple settings like prefix Network/Target/Timestamp...)