Hi Kevin,
Registration is not happening at all. By this, I mean to say there is no pop- up coming up to Select the Certificate from the Certificate store. Although, there is an Entry coming up in SMP Logs (I can say, a touchdown may be and after failing- giving an error as AUTH_PROXY <blah> <blah>)
For X.509, Config is pretty straight; right? Select the X.509 as an SSO mechanism in Backend and similarly, Select X.509 again for Authentication. Once done, edit the impersonator stuff.
And Yes, we are using Fiori via SMP (Auth type/ SSO type as X.509 Certificates).
KR,
~ Mahendra