Honestly, this would be a non-issue if mappers took a cue from iNiS style. The entire point of mirroring patterns is that the map is fair for both righties and lefties. Ouendan/EBA did this and osu!stream does this, because touch-based devices make it equally likely that either hand is used.
The case for not worrying about hand fairness in osu! is the fact that, even with natural lefties, the mouse is still on the right side. This is fine, but then the lefties are still stuck. A universal lefty/righty option could be an interesting addition to the game, since we probably aren't going to see frequent copypaste becoming popular any time soon. We have a lot of maps which are rotated slightly, in order to favour right-handed mouse usage by complementing the arc of the player's forearm. I like the fact that somebody's actually considering the accessibility of maps, as this kind of thinking can help novices, who won't necessarily commit to a game which makes them struggle.
If all else fails, go into the editor, hit ctrl+A and then hit ctrl+H to flip the whole map horizontally. This obviously won't be useful when playing for ranks, but it'll work just fine when playing for fun.