There are those who have asked for a way to transform the REMOTE_USER variable when it's used for display purposes (like in the enadis.sh CGI program). This can be perhaps more desirable when client certs are used with +FakeBasicAuth; the rather long identifying strings then used in the xymonpasswd (or comparable) file are a bit ugly.
AFAIK, Apache's mod_env will not modify standard CGI environment variables; so the CGI's would have to do it. If they checked if some optional RE was in a config file, they could use that to convert REMOTE_USER into something better suited to display than e.g.
/CN=CAcert WoT User/emailAddress=user-df14c361be72@xymon.invalid
(trivial example of what a free cert from CAcert might show up as); or there could be a file that just mapped REMOTE_USER values to display names.
Overkill, or worthwhile? :-)