I have two cases, it is very hard for me to separate them. I see them as redirection -problem.
# 1. I want to redirect all .*/feeds to mySite.com/feeds showing
# the content of mySite.com/Layout/feeds.html but keeping the
# url intact as mySite.com/feeds
# 2. I want to redirect all non-existent art.mySite.com/.* to
# art.mySite.com
RewriteEngine On
#REDIRECT pages that are not specifically redirected
#
# Alert! I am not sure whether this intervenes, sites not
# of that Regex -match are directed to the parametered url.
#
RewriteCond %{HTTP_HOST} !^www\.mySite\.com$ [NC]
RewriteRule ^(.*)$ http://www.mySite.com/$1 [R=301,L]
Case 1
#1. BROKEN: it directs mySite.com/feeds to mySite.com according to the
# above notice, apparently?!
#
# mySite.com/feeds and feeds.mySite.com must show the
# content on mh.com/Layouts/feeds.html but the url must remain
RewriteCond %{HTTP_HOST} ^www\.mySite\.com/feeds$ [NC]
RewriteRule ^(.*)$ http://www.mySite.com/Layouts/feeds.html [R=301,L]
RewriteCond %{HTTP_HOST} ^feeds\.mySite\.com/$1 [NC]
RewriteRule ^(.*)$ http://www.mySite.com/Layouts/feeds.html [R=301,L]
Case 2
#2. BROKEN
# art.mySite.com/feeds directs to Blogger -site...BAD. It is because
# art.mySite.com displays the Blogger content but it tries do display
# Blogger content also in non-blogger urls such as art.mySite.com/aoeu
#
# not doing anything or?
#RewriteCond %{HTTP_HOST} ^art\.mySite\.com/.*$ [NC]
#RewriteRule ^(.*)$ http://friend.blogspot.com/$1 [R]
# WORKS i.e. directs the art.mySite.com to the Blogger and keeps the domain
# but this does not handle the non-existing cases trying to do with (2)
RewriteCond %{HTTP_HOST} ^art\.mySite\.com
RewriteRule ^(.*)$ http://friend.blogspot.com/$1 [R]