Well on monitors that already support VSR you can enable it and then add a custom resolution of 3440 x 1440 @ 60Hz with reduced blanking via the Radeon aditional settings and then enable that res in windows.
For example my Asus MG279 monitor it is 2560 x 1440, using the above I can run a 21:9 res of 3440 x 1440 as per this screenshot.
Since you can't enable it you won't be able to do that but you may be able to add the resolution via the CRU app and apply it in windows, I don't have a 21:9 monitor to test it with.
I´ve tried CRU but I only get 16:9 resolutions like 1440p 1800p and 2160p. I want 3440x1440 and 5120x2160.