Você pode enviar o valor de REMOTE_USER na string de consulta enviada para seus servidores de back-end:
RewriteCond %{LA-U:REMOTE_USER} (.*)
RewriteRule ^/test.asp(.*) test.asp?userid=%{LA-U:REMOTE_USER} [QSA,P,L]
O LA-U significa olhar para frente. Do manual do mod_rewrite :
%{LA-U:variable} can be used for look-aheads which perform an internal (URL-based) sub-request to determine the final value of variable. This can be used to access variable for rewriting which is not available at the current stage, but will be set in a later phase.
For instance, to rewrite according to the REMOTE_USER variable from within the per-server context (httpd.conf file) you must use %{LA-U:REMOTE_USER} - this variable is set by the authorization phases, which come after the URL translation phase (during which mod_rewrite operates).