This is partly a suggestion, and partly a query to the DF folks about the best way to do it.
So, all this talk about vampires has me thinking about some of the stuff that Toady hasn't yet talked about dealing with: can't enter without invitation, repelled by the cross, by garlic, can't cross running water. I think Toady wants vampires to be procedurally generated such that all of the things involved in real legends are at least possible, but sort of randomly determined.
Of course, there's vulnerabilities, but it's not like anybody's going to have a +garlic halberd+ right?
So I was starting to think about pathing restrictions for certain creatures-- cursed creatures, for sure, maybe other creatures.
There's loads of awesome stuff you could do with this, that would lead to fun discovery in-game: like the vampires in your world can't stand fisher berries, that they can't stand images of two flasks or of Zuglar contemplating (because the image of the cross stuff isn't really appropriate), that they can't walk over running water, can't walk over running magma, can't go through an unopened door, can't walk over a piccolo-- you get the picture, it could be cool figuring out what you needed to do, and discovering some accidental sanctuary.
But the part that I was thinking about was the details. Could you do some kind of pathing cost? That would make it so vampires could be forced over garlic, but they'd take almost any other route in preference. Could you just ban a certain creature from a square, make him jump back and recalculate path? That seems like it might be problematic if there were two bane squares next to each other, with the vampire just pathing alternately between them uselessly. Could you treat bane squares as locked doors, impassible, not to be pathed through? Not sure how pathing works, if it would be difficult to notify each square of a "do not path vampires" every time you dropped some garlic on it.
And what about combat? Would carrying fisher berries make the vampire jump back, fail in all attacks? It would be uncommon enough in dwarf mode, but exploitable in adventurer mode-- once you learned the secret, at least. Maybe it should be exploitable? Should it just autofail vampire charges, as they can't enter your square? Or should it have no effect?
I think this has the potential to be really interesting, the sort of thing that makes a player feel smart when he or she figures it out and exploits it. It's not just about vampires-- i think bogeymen and clowns would both benefit from procedurally generated banes like this. I mean, how cool would it be to realize that your engravings formed a tiny square of protection? (Now all you have to do is get your dwarves to stay there.) It would also have the potential to draw flavorful things (crafts, images, etc) into gameplay. An amulet might suddenly become very important to a player, depending on what it depicted.