We built something similar to this a few years ago for our campus (we are a school). Hired an illustrator to do a watercolor of the campus on large tiles (maybe 8" by 11") and then scanned them in at high resolution and created a large image that we ran through OpenLayers [1] and built a searchable campus map. Neat project. Unfortunately, maintaining the watercolor (even with the tiles) as we changed the campus (e.g. construction, new roadways, new names for old POIs) was difficult, and we had to move back to just a PDF.

[1] http://openlayers.org/