I skimmed through this so apologies if I missed some more conversation around this, but I like the suggestion @Valki proposed about having an option to toggle between low and high res versions of the maps. It sounds like it's not feasible to automate that for how few maps will be impacted, but what @Jip said about making a decal and giving the people the option to do it manually sounds like a nice compromise there.
I'm also still not seeing the issue with having people be forced to download the maps before queuing. It's the difference between making it annoying for the handful of people that might be impacted vs every single person they try to queue with and the queue fails, which I feel has been happening more recently. The maps have to be downloaded regardless, how do data cap issues come into play there? I suppose it's different if they're just playing a couple games there's a chance there's less of a total download size, but not for anyone playing a lot of ladder/tmm. If it fails for them to download before queueing, it'll also fail during queuing which is a worse experience for more people. Maps are reused a ton throughout the pools. Do that for two or three pools and they'd have most of the maps for ladder/tmm anyway, and this would be coupled with the lower res download set.
Also, how hard would it be to implement what @arma473 is suggesting to improve the downloads?