Haven't resolved why superfaction doesn't want to work in my mod but does in vanilla / CI. Basic changes in strat to align them didn't work - could take rest of mod apart to find out why but after testing effects in vanilla I'm not sure it actually achieves what I want anyway ( & senate messages would be pain). Looks like most promising option is the non-playable, non-expansionist, faction's characters getting traits with
Effect MovementPoints -100
For duration that you can prevent any armies with captains being generated (or released on death) that seems to work to keep spare AI faction in place as defensive garrison. (so far, so good anyway)