The order flow will be very similar to what you have for foundations when you already have the resources : You select a bunch of units : the builders. Select the building from construction panel and place it. Once you'll have enough resources, the preview will become normal foundation and builders will be called in to build it. Since they're called with pushorderfront, they will actually also go back to previous task (e.g. farming) once done building. So it's almost the same to how it's done with vanilla construction  Still need to refine some details, but I make this kind of features trying to introduce neither technicalities nor arbitrary behaviors that's not user-defined... Although it's sometimes unavoidable to make some design choices. 

o.O still get err 500

Still down?? o.O

Who starts building, and when? Since having idle workers is not a good thing, did you code it such that workers have queued tasks that are done when resources are available? (I know I could test it myself, but I've barely played these last weeks).

This civ with this building set should be added to Millennium AD as well IMHO. P.S.: don't think i did not notice that the Egyptians opened a franchise there