SS will never offer exclusivity because they lack the technical structure to support it and have seemingly little interest in ever updating the website in any way. They still can't even get vector and JPG versions of an image on the same page. I have serious doubts that they have the capability to overhaul the site to the extent required to divide contributors into exclusive and non-exclusive groups, adjust payment schedules accordingly, tag images as exclusive, etc. That plus I doubt they would want to invest the money in it and I even wonder if they have the technical ability to do this sort of an upgrade at all.
I wouldn't hold my breath on an SS exclusive offering, and even if it did happen I doubt they will offer the $1+ per DL that a lot of people would require to make it financially feasible to go exclusive.
Exclusive or non-exclusive
by photo. When they fix the JPG / vector problem it would be a natural fit to add one more category at the same time.

Asking for people to be agency exclusive wouldn't make sense for the SS program.