It would be just as effective as a full on strike, without risking their jobs in anti union states. Especially if they made public newspaper announcements that this is what they will ALL be doing.
This is called “Work-To-Rule” (a form of malicious compliance), and is a very useful and effective tool in labor organization!
My dad did this! He’s a high school teacher, and he and all his colleagues refused to write college recommendation letters - which was dozens of hours of unpaid labor not in their contract - until they got a renegotiated contract, and it worked in like less than a week cause parents went apeshit. The education system would fucking collapse without unpaid teacher labor, and that’s a huge source of leverage