Splitting the queues into open map and closed map queues is a very simple solution which would solve a lot of problems for players. It is very common for players to have large skill differentials between their open map and closed map gameplay. This leads to players strongly favoring the one in which they are more skilled. It also leads to very unfair matchups where a player forced to play their weaker map type is almost guaranteed to lose. Players who find both map types enjoyable will still benefit from more accurate matchmaking even if they themselves are equally skilled in both. Unlimited bans would be very desirable, but designing a match making system that supports such a thing well would indeed be very difficult. Splitting the queues, however, gets us close enough and would be very straight forward to implement.
For the most part, players that strongly prefer open or closed maps no longer have to play maps they hate. Players who like either type can queue up for both and get a fair matchup with a player that has a similar skill level for the map type, instead of wasting time against someone who is way too strong or way too weak.
At the time of the initial split, each player would be given their current ELO in both queue types, and as they play their ELOs will correct themselves.