Profiles on sic.pm have a metadata section similar to what is present in Mastodon. Great for this purpose.
It currently seems that this metadata is not returned by the webfinger implementation. The developer behind sic.pm is going to look into this (https://chaos.social/@epilys/108002199629152379):
awesome, will look tomorrow at webfinger spec on how to make this compliant by serving the required public key etc fields from the user profile! Maybe you can generalize on this to support all webfinger implementations, though I think it's not a very widely used thing. (Mastodon uses it for sure)