The Royal Order of the Holy Mackerel has loads of extras for members to find and decode, such as the codes at the end of each meeting, which when solved, either show a hint to the next meeting, or a question related to the topic for members to answer.
Such codes are also found on the show's official site, and its Tumblr page. If decoded with the Bill's Substitution Cipher, and the Alchemy Font Cipher, it reads: "THE GREAT OWL PROTECTS US" "TRUST NO ONE" "THE DARKNESS IS COMING".
You can also find a code in the first Puzzle Video Challenge, and when solved with a decoder ring, it says: IN MABEL'S GUIDE TO ART, WE LEARN DIPPER HAS GREEN EYES!!, which is a mistake, because Dipper's eyes are brown.
During a few of the first meetings, Douglas MacKrell came upon a suggestion to make backwards messages at the end of his videos, which came from a person on Facebook. In the meeting "Bill Cipher's Wheel", there is a backwards message saying: "The Riddles are Double-Encoded", which is a reference to that week's challenge which was corrected due to being too hard for the new members.