This has been brought up before, although with different intentions. I honestly don't think a survey would help. There wouldn't be a great way to expose that information to users who come here the first time, and anyone who's been around for a while has a feel for what types of questions get good answers. Also, a more accurate way to explore the scope of the site is to simply ask questions and see what kinds of answers it gets. The ability to get people to participate in the survey also greatly hurts its chances at being valuable. Most people would miss it, so any specialized knowledge that one or two users have is likely to be sampled out from the start. It's also hard to quantify likelihood to answer questions on a topic, even if you know they have the knowledge.
If there was going to be a survey, Stack Exchange itself would probably need to be involved in order to be effective. I could ask our community team if that's something we ever do for the smaller sites (I know there's a survey for SO users once a year), and whether or not they think it would be a good idea in this case. However, my guess is that their answer will be the same as mine - that asking questions is far more effective at defining scope.