I really hope that Ask Ubuntu will eventually have a separate avenue for becoming Ubuntu members. Not only does it puts us in par with other official support platforms like IRC and the Ubuntu forums, who already have such platform, it also motivates the contributors to do a little extra towards Ubuntu. I consider it to be a win-win situation.
I don't think this would be in violation of anyone's preferences but rather, act as an appreciation of users' contributions to Ask Ubuntu.
While you can always apply through the general route, it is pretty tricky to explain to the community council what sort of a platform Stack Exchange is. It is also not obvious for them to see beyond the number of answers you have so far. While, we the commoners of Ask Ubuntu would exactly know how useful a certain user is, that user would lose that advantage if he is to apply through a general route.
I don't find the argument that contributing to Ask Ubuntu is not just enough particularly convincing. We already are a thriving community and one that we should be proud of. While I had activity across a variety of community, which helped my Ubuntu membership application get approved, I would like to point out that Ask Ubuntu was a big part of my contribution.
In short, we should go beyond just encouraging Ask Ubuntu contributors to become Ubuntu members and set up a community council specially for that case. I doubt anyone wouldn't want to have that.