Hi CK - I'm the OP and I would use custom scales exactly how Will explained.
"I can't speak for the op, but a custom scale would allow one to program a
specific harmony for a melody where the harmony jumps out of a key
(accidental notes), in the case where one might want a harmony of a
third for the majority of scale steps but a forth for the tonic, or a
case of a melodic minor scale in which ascending and descending
intervals are different"
I have an Eventide H8000 and it has Custom scales of course,however, who wants to lug a rack around if it were already in the Kemper.
I see your Kemper as a VERY PORTABLE all in one solution, it's that good (thanks to you)
I intend to purchase an integrated stereo amp for the Kemper so I can use it into a guitar cab when I'm NOT using an FRFR system (RCF322a or Mackie SRM450) With that said based on what's onboard already in the Kemper having custom scales would make it a total all in one solution for ANYguitarist. If getting this option costs money I'm prepared to pay and don't expect it for free.