Since n8n doesn’t really do domain based routing, to be able to support a different domain for forms would require a reverse-proxy to handle the additional domain.
But if there is already a reverse-proxy in place, one could configure it to handle the form specific base url, rewrite the path/hostname, and then forward the request to n8n.
This won’t use the “correct” url in the n8n editor, but most of what feature request asks can be implemented outside of n8n IMO.