Hi,
You can control (via user exit or BTE 2310), which FIKO text will be read by standard logic (the text name has to be passed in FINAA-NAMEP or FINAA-MAIL_BODY_TEXT - see subroutine CHECK_MAIL_TEXT in RFKORI90) so you could define texts, including appropriate footer, per company code.
Inserting variable values is trickier, because, unless you are willing to do implicit enhancement in subroutine SEND_MAIL_WITH_ATTACHM of RFKORI90, there seems to be no place to do it other than the same user exit or BTE 2310... If you can solve the challenge of passing the data you need (reference number etc.), I guess you could try to read the text in the exit, do the variable replacement and then update it (no commit though!), but that opens up the whole question of concurrent processing, and is not "nice" from the performance point of view...
If I had to do it, I'd define a FIKO text per company code and do the variable replacement via an implicit enhancement at the beginning of SEND_MAIL_WITH_ATTACHM of RFKORI90...
cheers and good luck
Jānis