According to this update, “Made an update to ensure that the “Zealot” achievement is unlockable.” There is also a link to this post which says, “For any achievement labeled as multiplayer, unless matchmaking is explicitly stated in the description, it can be earned in custom games as well.”
The description of Zealot is “Halo 2A MP: Win 10 multiplayer games as an Elite.” No mention of matchmaking.
So I hoped that Zealot was now obtainable in Custom Games. However, I have not been able to make any progress on it in Custom Games. Since I don’t like MP anyway and it sounds like matchmaking is still buggy, I’m trying to avoid MP as much as possible. Thanks for making most of the MP achievements obtainable in Custom Games. I hope Zealot will be added to that list as well.