In my opinion, a viable solution can be found. As I understand it, we can check who among the participants voted for random maps and who voted for custom maps (if this isn't possible, we can ask them to express their opinion on the forum or via ICQ to the judge).
Then, we create two types of rules in the regulations: 1) for games with custom maps, and 2) for games with random maps, plus two types of groups. After the group stage, we conduct a poll among those who made it to the playoffs and hold the playoffs according to the majority's choice.
In my opinion, this is the most optimal option.