modify URL with mod_rewrite or hook

i fixed it using the apache proxy module:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /

RewriteCond %{QUERY_STRING} !location
RewriteRule ^(boston|newyork)/$ /index.php?location=$1 [NC,QSA,P]

RewriteCond %{QUERY_STRING} !location
RewriteRule ^(boston|newyork)/(.*)$ /$2?location=$1 [NC,QSA,P]

RewriteRule ^index\.php$ - [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

</IfModule>

the [P] option does the trick.