Endpoints to test various SSL/TLS/X.509 stuff would be super helpful. Call would returning a stock HTTP response body, but with different changes to the SSL/TLS handshake (you will have to use different host names to do this)
We've discussed doing an "TLSBin" as you described (and still might) but it's a lot of work. We'd gladly sponsor one! This is something I wish Qualys/SSL Labs would do.
https://expired.x509.httpbin.org/ - returns an otherwise valid cert that has expired
https://wrongcn.x509.httpbin.org/ - returns an otherwise valid cert with a different CN name
https://sslv3.tls.httpbin.org/ - only negotiates an SSLv3 connection
https//3des.tls.httpbin.org/ - only negotiates to use 3DES as the cipher