While we haven’t yet closed the loop and hired anyone from this method
That's the key measure. To be honest, this is such an old technique that it doesn't seem especially cool or interesting. We've found writing good blog content and working on open source to be effective at finding good people.
This was cool in 2008, maybe even earlier. It's not anything fancy anymore. I was able to view headers of pages when I was 15 and it is probably the worst way of getting applications. Instead, put up a few problems you had while development of your product and get solutions to your recruiting email. See if people can come up with similar or better things. This will make you find better engineers. Headers will get you script kiddies.
How about an X-Human recruiting message with a LINK to some problems you have?
Very few people will see that link and it would be your onboarding process for new hires and it will save you time reading resumes and interviewing candidates.
That's the key measure. To be honest, this is such an old technique that it doesn't seem especially cool or interesting. We've found writing good blog content and working on open source to be effective at finding good people.