Hello Anthony,

I tend to agree with Anatoly in that this does seem to be a pretty niche
requirement and adding another configuration option may not be desirable.

Might it be possible to implement this functionality using a custom
login action? There is some information about creating new actions at:

 
http://roundup.sourceforge.net/docs/customizing.html#defining-new-web-actions

and more specifically some login action examples listed under 'security' at:

  http://www.roundup-tracker.org/cgi-bin/moin.cgi/CustomisationExamples


You also mention you are hitting this because the REMOTE_USER variable
is not being passed through the reverse proxy. Might is be possible to
do something like what is mentioned in the following serverfault answer
to work around the issue without needing to make any changes to roundup?

  http://serverfault.com/a/231577