I also stumbled across this old mockup.
Old? I was barely born when this was made :)
We put quite a bit of time in exploring the whole Buildings on the combat map issue. The thing is it turned out to be really tricky to implement multi-tile objects with our engine (2D, height levels, hex tiles etc).
Our last attempt looked like this:
We had to cut the objects, so that no element is wider than one tile and then piece them back together in the combat map. It was a hassle :( So we finally decided that our time is spent a lot better on improving the gameplay and adding content instead of finding a dodgy workaround to implement large structures.
Apart from the technical issues, the gameplay effect of large objects would be very doubtful as they would basically just clog the map without adding any strategic depth (even though of course they would look pretty cool).