Eu acredito que isso deve dar certo:
RewriteEngine On
RewriteBase /
# Turn SSL on for /user/login
RewriteCond %{HTTPS} off
RewriteCond %{REQUEST_URI} ^/user/login
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L]
# Turn SSL off everything but /user/login
RewriteCond %{HTTPS} on
RewriteCond %{REQUEST_URI} !^/user/login
RewriteRule ^(.*)$ http://%{HTTP_HOST}/$1 [R=301,L]
O texto acima fará o seguinte:
1. User types: https://yourdomain.com/user/login - no redirect
2. User types: http://yourdomain.com/user/login -> redirect to: https://yourdomain.com/user/login
3. User types: https://yourdomain.com/somerandomfile.php -> redirect to: http://yourdomain.com/somerandomfile.php
4. User types: http://yourdomain.com/somerandomfile.php - no redirect