Just tried it myself and yes very quick to put in but again I do wonder what the Warden anti-cheat engine would react to this.
I opened the Overwatch.exe in a hex editor and searched for 39 8E E3 3F (hex for 16:9 aspect ratio) believe i found that hex string twice in the file. I changed the second one (about 3\4 the way through the file) it found to 8E E3 18 40 (hex for 21:9 3440x1440) and bam main menu was in 21:9 ratio. Didn't go further than the main menu because again don't how Bizzard's anti-cheat would interpret this change so didn't want to risk it.
If you need a different hex value to change for your res then look here
http://www.wsgf.org/article/common-hex-valuesEDIT: Sniped to the answer, still nice to know I did it the same way
although @ggouki did you replace both instances of that hex string or just the last one like I did? Just curious.