Not sure about /install/, but should work too.
here is my solution on
http://mediabox.lv/qa/
in .htaccess
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /qa/
RewriteRule ^qa\/index\.php\/rewrite-test qa/index.php/rewrite-pass [L]
RewriteCond %{REQUEST_URI} ^(.*)//(.*)$
RewriteRule . /qa/%1/%2 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* /qa/index.php?qa-rewrite=$0&%{QUERY_STRING} [L]
</IfModule>
it looks like error couse was because there is an unescaped dots and slashes qa\/index\.php\/ should solve problem