Ask HN: Best way to hire a CTO in Silicon Valley?

My development firm, based in NYC, has a client in Silicon Valley. They are seeking a CTO to work with us before the site launches and to grow an internal team thereafter.

We're in a niche where this is pretty common, and the development idiom of the industry is such that a transfer of knowledge should not be a burden if we do our jobs right. Also, the best list of jobs that I know of in this niche is a Google Spreadsheet.

Normally, my answer for them would be to plumb their network, post on StackExchange and LinkedIn, etc., etc., but they have fairly specific needs around experience and prior knowledge. Also, they need to keep some details of their enterprise confidential before they launch, since they're fine on funding and would not benefit from pre-launch publicity.

Is using a recruiter unavoidable in this case? If so, who are the best? Are there alternatives? What's the best way to advertise a great job while withholding some details?