Comment 32 for bug 895118

OmegaBolt's system is probably the best suggestion for this issue.

narfnin's suggestions about EngineeredFrom= stuff also sound good. You could make systems in which you receive certain elements of a unit with this method (like prism weaponry from the Prism Tank) and allow your engineers (let's say you're Soviet) to construct a completely unit based on the prism tank etc.