Hi! The WordPress accessibility team did some extensive testing of Select2 with an eye towards whether or not we could make use of it in the WordPress core. Unfortunately, we found a lot of issues that need to be addressed before it's a reasonable candidate for us. You can find the results of our user testing here: https://make.wordpress.org/accessibility/2015/09/07/accessibility-usertest-select2/

We'd love to work with you to resolve these issues - let us know what the best way for your workflow would be for us to move forward on this! Thank you!