Can we/should we promote Area 51 proposals in the tag wiki?
I would say no, especially if this would be done for every proposal without to make a difference between proposal, and proposal. For example, would you promote a proposal that has been proposed 2 weeks ago, or just proposals that are close to be a beta site?
It makes more sense to do it for a site that is in beta phase, but even in that case there isn't anything that assures the site will pass the beta phase.
The question that comes in my mind is the following: Is promoting a beta site in a tag wiki really helpful? This has been done with Drupal Answers, but I am not sure it had any effect. Did users on Stack Overflow ask more questions on Drupal Answers because drupal, drupal-6, drupal-7 contained a reference to that SE site? I doubt it has been so, and mainly because few users really read all the tag wiki; of those users, the users with low reputations are the minority, I guess.