nginx is connecting to php-fpm over fastcgi, using the standard
/etc/nginx/fastcgi_params in the location block.
When connecting to /.status (php-fpm.ini::ping.path) from the command line with
cgi-fcgi -bind, the result comes back as expected (X-Powered-By set, response body, etc).
When requesting with nginx, the result comes back empty (X-Powered-By set, no body length or content). nginx returns 200, since it got a "valid" response.
Watching over tcpdump, I've isolated the requests to parity in their FCGI headers (minus user-related env variables still set by the shell.)