I've not really looked into this but I certainly have had a roman and non-roman faction in BI 1.6 (modfolder) with the superfaction line under both entries in DS without any problems (except all that darn senate stuff popping up).
I assume from way you described that you meant without having romans_senate modded in? Apparently my hypothesis about needing them may be wrong. But in that case I can't understand my results in RTW under BI.exe.

I tried again later in vanilla BI and that would accept lines
faction empire_west, fortified caesar
superfaction empire_east
and launch game but it didn't appear to have any effect (no senate tabs etc), not sure if other factors, non-expansion, fixed alliance etc worked or not.
Tried same lines again in my mods modfolder and got crash again, brief moment of inspiration as was using empire_east etc. (which aren't shadowed in mod) I re-instated the rebelling characters active line and it loaded once, but going back to exact same situation it now won't work. Have tried applying line to other combinations of factions, shifting culture designations around, deleting or not map.rwm and changing or deleting diplomacy settings but nothing is letting a variation of the mod descr_strat with 'superfaction...' line in it load reliably.

As to what I'm trying to do, I just want a faction to act as guardian for one particular city. It will have superior units so I don't want it to move out and mug everyone else - but I do want other factions to be able to get to it later in the game, so it can't practically (or historically) be inaccessible. Saw someone (seasoned alcoholic?) post that you could do
faction egypt, fortified caesar
superfaction egypt
to effectively stop faction moving so wanted to experiment with that, but as can't currently get superfaction in mod at all, I'm not getting very far!

I think I'm having one of those 'unique' moments....