The minimap marker (and screen title) would help on direct warps to identical screens (and for names of building interiors). Probably minor, but that's the idea.
The reason for having a string without a guy is that setting the guy to "Empty" prevents several kinds of secrets from functioning.
Do you know what having an Empty guy breaks?
I still do not understand what you mean by setting the minimap marker on a per-screen basis. You want to set it to something other than the true position? (I think that you can do that with a tiny script in 2.54.)
I also have ZScript functions for changing the dmap title, and other strings, in 2.54, so, you could use those to change them for special rooms or such.
I would rather spend the time making the DMap intro something that you can see, than something tied to screens.
I might be able to fix the Empty room issue, or add some extra guy types.