I really like this idea!
In terms of mapping, the style of the HQ is determined by the number assigned to it on worldbuilder. Every HQ is the same shape, with a different style (rural, industrial, urban or Holland), and then the exterior scenery such as flagpoles, sandbags and wooden walkways are added in separately. You may notice that regardless of playing US or Wehr, the sandbag layouts will be exactly the same, however the Soviet Union HQ's have no defenses (last I checked anyways).
Brits are the exception to the rule, and used to have a separate flag called "Commonwealth starting position", which replaced the HQ building and spawned the Truck on that spot instead, however this is outdated and is no longer required. The truck will sit where the HQ building is supposed to be.
So, if anybody understands how, I can't see why a new model for the HQ building can't be made (similar to the differences in skin between the rural and Holland HQ's, for example), or have a spawning flag similar to the Commonwealth flag.