Webform submissions and og_group_ref
Interesting question/challenge (+1 from me ...). What you described about the "is group content" makes those webforms indeed unreachable for anybody who is not part of that group (so that's a "works as designed" ...

However, how about using the Rules module to complete your puzzle? Haven't looked at all the details (yet), to see if it can actually be done (I'm pretty sure it can ...), but I'm thinking of using a rule that looks more or less like this:

Event: when Webform is submitted

Condition: If membership for group is requested in the submitted form

Action: Send eMail to an authorized user to accept / reject the request