Thanks chaps, but I know how to do it, found list of files on Dromikaites post a while back
(https://forums.totalwar.org/vb/showthread.php?t=59665)
and have already got a mix of RTW & BI faction names working under BI.
(BI hardcoding has all RTW functionality, so no suprise that, that works)

It is a long process to change internal name, as soon as you take a faction name out of sm_factions you'll get KTM or CTD until you remove all other references to it in the text files. Yes I have got RTW unpacked can't remember if it was necessary for this specifically.

It shouldn´t be so difficult of writing the new faction names down on a sheet of paper...
I know I can write cross-reference list or comment the files but thought it would be better in the long run to resolve this and files would be easier to understand in case mod ever does get posted (need a flying pig smilie here).

Question was only whether anyone had changed an internal faction name to one that didn't exist in either game. i.e. are the possible names hardcoded, being lazy thought I'd ask before I tried it.