This is actually like the lore for the current infested marines. The idea for them is that the infestation creates a symbiotic organism made to use the technology as a base to build on top of, which allows the small amount of biomass in a human to be enough for a lethal zerg organism. Personally, I thing that the projectiles fired from the infested gauss rifles are dissolved parts of the suit that got replaced with the organic shell. The only in game advantage the infested marines have is health regeneration, with all the disadvantages being explained as a result of a bad hive mind connection and constant resisting of the victim. An infested structure would not have that problem, and to make it, the zerg would have to make a strain capable of correctly connecting with machines, which requires that the same strain could grow over a bunker with a strike turret. Even better, infested hellbats, SCVs, and thors would be made available, with the only adjustment being a growth layout to put the parts in place. Infested SCVs would be a LOT faster than usual, with the possibility of actual flight. That would open cliff mining, and with the fortress being mobile you could mine from a cliff without the need for buildable space. Also, the thrusters on the fortress from the command center are still there, so a cliff jump would be reasonable to have. Perhaps to get the infested fortress you could need an infested SCV rearrange the internal parts to fit the organism's layout, with unusable machinery being dissolved for the regenerating carapace.