Just thought I'd share the final functions I settled on for isometric coordinate calculations. Unlike most of the others I've seen around the web, these account for:
* Origin offset
* Tile-based position offset
* Varied scale
* Any aspect ratio (not just 2:1)

While ZedZed required only simple directional pathfinding (zombies are stupid, so advanced pathfinding is not needed), Demon Keeper required some more advanced techniques to facilitate targeting.