Aqui está o que finalmente funcionou:
# Force HTTPS for entire site
RewriteCond %{HTTPS} !=on
RewriteCond %{THE_REQUEST} !^[A-Z]+\s/directory/page.*$
RewriteRule ^(.*)$ https://www.mysite.com/$1 [R=301,L]
# Force HTTP for a given page
RewriteCond %{HTTPS} =on
RewriteCond %{REQUEST_URI} ^/directory/page.*$
RewriteRule ^(.*)$ http://www.mysite.com/$1 [R=301,L]