Use WebSVN as default HTML view

What about using mod_rewrite? You could do something like this:-

RewriteCond %{HTTP_USER_AGENT}          !^SVN.*
RewriteRule ^/svn/(.*?)(/.*)$           http://websvn.host/websvn/listing.php?repname=$1&path=$2      [R]

This should redirect any client that doesn't have a User-Agent string starting with 'SVN/' to a WebSVN installation at http://websvn.host/websvn/, and leave queries from SVN clients alone.