Hm, would you want to delegate to the `pgp` one to prevent any other login mechanisms from being used? One of the nice things about indieauth.com showing multiple options is that depending on the device you're logging in on, you might want to choose a different option. For example I usually use GitHub or GPG login when I'm on my main computer, but use Twitter from my phone.
I can definitely see value in wanting to limit the options provided by indieauth.com to a subset of the rel-me links on your site. (Maybe I want Twitter listed on my site, but never want to use it for login.)
What about using the query string to indicate the supported providers?
`https://indieauth.com/auth?providers=github.com,pgp,sms` etc. In that case, indieauth.com could even present them to you in the order given.
Similar to https://github.com/aaronpk/IndieAuth.com/issues/112, if only one is set then it could redirect immediately instead of making you click the button, which would be a better user experience.