Suggesting to ask questions on Drupal Answers should be limited, and it should not be done in a comment for a question about Drupal asked on Stack Overflow. For what I can see, most of the users (who don't have a high reputation on Stack Overflow) would just ask the same exact question on Drupal Answers, with the effect of having the same question in two sites. This should not happen, and the OP should be informed that he can ask to migrate the question to the moderators of Stack Overflow.
I understand the need to promote Drupal Answers, but at the same time I think users are free to ask the questions where they prefer. I think that questions that are related to Drupal, but which are not about programming, should preferably asked on DA, rather than SO. At the same time, I think that users that ask most of their questions on SO because most of the questions are about PHP, and not Drupal, should not be forced to ask their few questions about Drupal on DA.
To notice that the "drupal" tags on Stack Overflow has a reference to DA. It is not enough for redirecting users to DA, but at least there is a reference to DA that can be read from Stack Overflow users.
Most of the promotion should be done outside the Stack Exchange network, which anyway promotes DA showing in a block questions asked on DA. If you want DA promoted on SO, ask more questions on Drupal Answer; the more questions are asked, the more probable is that a question on DA is shown in another SE site, and that a new user registers an account on DA.