A good solution how to make plane cons build things:
* There is a unselectable builder somewhere
* A button let you select the otherwise unselectable builder
* If you assign build orders to this builder, a building-target is spawned.
* The plane-"con" can attack building-targets and the building targets build the building.
* The build-target counts the damage it gets from plane-"con"s and build the building using 1 metal for each damage point it received from plane-"con"s.
* It is possible to substract any metal/energy amount as a penalty for free build power, limit the build power to something like 2m/s per building or alter the build power dependent on the remaining damage->built value.
* If the building is destroyed or finished, destroy the attack-target.
* If you issue a repair order, this repair-target only drains energy - make sure that only one build/repair-target is available at a time.