When we land in Treasure Island, we're immediately engaged by the NPC Pirates, a crew composed of Pirate A, Pirate B, and Pirate C. Pirates A and B are regular Archer mobs but Pirate C is a Caster with infinite Guts. After that battle, Pirates A and B are never seen again, but Pirate C sticks around to say he's having a great time watching our adventure and thank us for being so entertaining every now and them. But the wording in key parts is ambiguous enough to allow the "he's here to see Douman" interpretation.
One of the treasure dungeons we explore during the adventure is decorated like a Shinto shrine and one of its puzzles involves a portrait of Sei Shounagon, which gets the characters wondering what is that doing in the middle of the Caribbean.
In the epilogue, Achilles they didn't discover who created the Micro-Singularity (since Christopher and Douman were competition in the treasure hunt, not the organizers) so he goes ask Chiron for a list of synonyms for "mage". After hearing all the words, Achilles goes "Hm, conjurer starts with a C... Pirate C... Perhaps?".
Then the B side of the event is Kama trying to prove her villain status by arranging 4 challenges themed after the Shishin, meaning Treasure Island can function as an onmyou map.
So, Seimei's face: