Eu descobri essa maneira de conseguir isso com o mod_rewrite do Apache habilitado. Fragmento de .htaccess
RewriteEngine On
# remove trailing slash
RewriteCond %{HTTPS} off
RewriteRule ^(.+[^/])/$ http://%{HTTP_HOST}/$1 [R=301,L]
RewriteCond %{HTTPS} on
RewriteRule ^(.+[^/])/$ https://%{HTTP_HOST}/$1 [R=301,L]