There are several ways to address this. One is to think of different activities, not competition oriented, to engage high school kids in CS related issues. In the CS4U days that Shai organized in previous years, we have had a reasonable percentage of high school female participants.
Independently of that, we may wish to talk to Troy about the possibility of employing "affirmative action" in the selection of students that are invited to that second stage.
The problem is what to ask them. "Why don't you choose CS" is what we really want to know, but we need more guidance than that so that if they answer "it's not cool", we know how to follow up. This will require a lot of prior though on our side, to make sure we make the most of such opportunity.