Some rares put a line in chat when they spawn. In Mechagon the Arachnoid Harvester for example.
Would it be possible to also show an alert if this happens regardless where I am in Mechagon?

Seems technically possible.

I think it'd just be a matter of watching CHAT_MSG_MONSTER_YELL for when its arg2 is the name of a mob we know of in the zone.

It'd be unhelpful for mobs which have a variable spawn location -- e.g. if Gear Checker Cogstar spawns.