I had GRAC on the Amiga which had a very similar system to how you describe, cdb. (I was aware of GRAAL but never tried it. Some of the games I saw made with it were quite cool though.) I want to do that kind of system (the other way with a walk zone layer image is a possibility though, I know the popular AGS system does it that way). I suppose the number of walk zones on a single screen would be small enough I could just pre-generate a matrix that says which zone to go to first to get from any zone to any other zone, and it just consults that each time until it reaches the target.