Hi Hannah,
Thanks for letting us know that you would appreciate such a feature in PBE. In order to maintain the generality of the framework, I would implement this as a location-specific repair cost adjustment factor. In a building, locations directly map to stories, so you'd be able to assign a factor for each story, or a range of stories as you see in the excerpt from the P58 documentation.
This seems straightforward to implement and I added it to my plans for the next minor release that is due in the end of April.
Adam