To be sure we’re on the same page, are you trying to allow your users to verify themselves on Mastodon through your Discourse site by doing something similar to what’s outlined here: Get verified on Mastodon with your website | Opensource.com?
If so, maybe prompting users to add the link to either a regular post, or to their bio would work. Either of those would show up in the site’s crawler view. I don’t think it’s possible to override a crawler template from a theme component: Header Submenus - #134 by awesomerobot.
The issue I’m running into is that Discourse automatically strips the rel="me" attribute from the links. Unless there’s a work around for that, the attribute might need to be added to the AllowLister via a plugin. I’m not up to speed on Discourse/Mastodon integration, but this might be something that’s useful for other sites.