If anyone is looking for a solution here, this is the workaround we figured out and are currently using in our dev environment.

We created a custom "Recipients" field in the Case table, and updated the Create Case step in the "Create Case for matched or unmatched user" subflow to write the Recipients from the email to this field. Then we create Assignment Rules in sysrule_assignment that set the Assignment Group based on a string in the custom Recipients field.

Hope that helps someone, let me know if you've come up with a better solution.