I am wondering if there is a way to via the classlist upload or other batch process to assign select students extra time (i.e. 1.5x or 2x the normal allowed time per ADA/504 accommodations) without us having to manually edit each assignment that a student is assigned to?
This doesn't exist currently. I have entered it as a feature request (https://github.com/openwebwork/webwork2/discussions/1865). If you have thoughts or specific requests about how this should work you can add them there. There is no guarantee that it will get implemented in the immediate future, but having more detail about the desired behaviour would help the developers get it right if it does get implemented.
In my use case this would only apply to timed sets (i.e. tests where a time limit is set in addition to the due date), as our accommodations don't apply to assignments (e.g. if a student has a week to work on an assignment, then they don't get extra time). If you want this to apply to assignments as well then that would change the implementation.
I would like to think that a field in the upload for classlist would the best. Your usecase for time sets is what I also envision.