It is indeed something only the ISP can do. Reverse DNS authority is delegated per octet. For example for IPv4 address 188.8.131.52 the reverse DNS record name is 184.108.40.206.in-addr.arpa. That is part of the 92.54.185.in-addr.arpa zone. The holder of 220.127.116.11/24 is the one who maintains that zone. That will be your ISP. If your ISP doesn't let you provide ...
Based on https://httpd.apache.org/docs/2.2/mod/mod_ssl.html#sslprotocol:
This is a shortcut for "+SSLv2 +SSLv3 +TLSv1" or - when using OpenSSL 1.0.1 and later - "+SSLv2 +SSLv3 +TLSv1 +TLSv1.1 +TLSv1.2", respectively.
So first, there would be a need to know which OpenSSL version you use.
Let us rewrite your statements:
1) before OpenSSL 1.0.1:
I think your logic is sound. If you only have access to the server providing the iframe content, I don't see how you can run a reverse proxy. I can see the client website going to for example, localhost:3000, and have nginx or something reroute that to your server, but without access to the client server I don't see how you can do that.
The only other idea ...