So we figured we might host one of those faction within our claimed land. As long as they store all valuable within our secure base, the fact that their claimed land becomes raidable doesn't really matter. Claimed land had to be at least one block apart from each other. So we had to find a bedrock tunnel: On their claimed land there needed to be free space within the bedrock level to get down to z=-63. Then on the single unclaimable block between our bases we needed bedrock on z=-61 and bedrock on -62/-63 on both sides. Then on our claimed land we again needed a way to get up from z=-63. Basically a single block long fully bedrock-enclosed tunnel. Took a while to find that, but we managed to set it up and caused quite a stir with other players and the server admins once our hosted faction got raidable without consequences :-)