I guess you have a webserver there on your pihole?
Create a vhost for reddit.com, add reddit.com to your hosts file to point to the webserver (or setup a stub in the dns to the vhost webserver), and do a redirect to old.reddit.com on your webserver vhost.
This is probably the easiest (simple http server and a php file) and most reliable (https can be redirected to http to avoid any ssl headaches) way to accomplish what the OP wants.
