If you have a website running under a server with a name in your GNS domain and you want to link to a server with the name 'www' in a different GNS domain, you'd add a PKEY record for that GNS domain to your GNS domain (eg. called 'foo'). You can then link to https://www.foo.+/ and it will DTRT - someone browsing your site under the name www.jonathonw.gnu will resolve the link destination as www.foo.jonathonw.gnu.
Or share a name with another user?
If you want to share a link with someone I guess you'd share the .zkey version.
A greasemonkey type script might also be able to apply the right fixups.