City guards can be used for this purpose, and they pretty much implement all
the ideas you mentioned (you can set desc, emotes etc.).

A lot of people have already told me this. And I did know that guards
already do a lot of this. However, while there may not be any need for
any specific addition to batmud, my orginal idea was to have something
a little more convenient than having to buy a safe location and guard
and, importantly, to have a mob that behaved (emote wise and damage
message wise) differently from a plain guard.

I assume that my clarification doesn't make my idea any more worthy,
so hopefully they'll be no need to point out why it is still not a
particularly great idea (if anyone does, I'll make sure to add a bunch
of worse ideas to the ideas group, so there).