Thanks to you both.
I ended up copying the "LargeHull6" type in GC2Types.xml, pasting it back in as a new entry, renamed it to "StarbaseHull0" with the Tech_Requirment changed to "ImpossibleTech" so it couldn't be used to build actual ships, then modded HP and sensor range.
I then went back to GC2Ships.xml and changed the Starbase entry's Component tag to point to the new StarbaseHull0
So far, looks to be working as planned.