Kalon seems to be extremely close to a solution.  Perhaps  you should be able to  assign your image to more than one gallery,and subscribe to more than one gallery. Then the critique group will be a gallery. You assign your image to Poser and the critique group and you subscribe to both.
Advantages:
-you can choose to not have some of your images critiqued without making a fuss about it.
-You still get poser traffic
-If you use poser and vue,  for example,your image can "belong"  to both of these galleries and get the traffic from both (I fear putting it in mixed media will get the traffic of none)

Surely this cant be such a big programming task. No need for new page layout. No need to maintain  a list  of group members. Just defining an additional gallery and checking assigned galleries in a loop rather than a single item.
(and while you are at it, enable more than one genre as well)

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/