Specifically, I'd like to know if the proxy_ssl_certificate and proxy_ssl_certificate_key directives can support RFC-7512 PKCS#11 URIs, or whether they're hardwired to be just local file paths.

With my private key in hardware, I'm looking for the ability to point nginx to something like:

location /upstream {
proxy_pass https://backend.example.com;
proxy_ssl_certificate /etc/nginx/client.pem;
proxy_ssl_certificate_key 'pkcs11:type=private;token=some_token;object=username%40example.org';
}

Cheers,
Erik van Zijst