I question the validity of the phrase "voluntary slavery". I would also argue that the context (USA-based company labor dispute) is enough to anchor the term "

I question the validity of the phrase "voluntary slavery". I would also argue that the context (USA-based company labor dispute) is enough to anchor the term "slavery" to the very recent and very extreme United States enslavement of Africans. Using the term "chattel slavery" may be a serviceable method for communicating that you are not talking about "that slavery".