I would recommend doing this with UserRoles. If your new feature is a new Module, utilize the secutiry to give only those User Roles you want the necessary Module Roles to activate the feature. For example, you could create a 'Tester' UserRole, and give onle the Tester role the 'User' ModuleRole that can use your new functionality. After the testers are satisfied, you can add that ModuleRole to all the UserRoles that are appropriate.
I have never done A/B testing using UserRoles, but I have assigned a random decimal between 0 and 1 to accounts, and those that were .5 and above went to flow A while those below went to flow B. You could extend this by checking for a certain userRole in your Users. If they had that 'Tester' user role, then you could route them to an A/B picker while non-tester users are sent only to the existing functionality.
Hope this helps,