I've done it before but just to be annoying or catty. You didn't hire me but I've been getting notifications from linkedin for your endless reposting of the same job ad over the last 6 months. What are you looking for in a candidate? What's been holding you back from making that hiring decision. I'm not interested myself but I know a few developers who might be interested.

That's probably the only way I've gotten solid feedback. This would not work at a larger firm. You slowly realize they are trying to find someone cheaper or someone with specific experience that they wouldn't normally share.

Advice become a recruitor and don't make it about you.