Idk, how would you work to alter the play field if you ran a union? I think I'd try to make it harder to compete with union members, and to improve the transparency and standardization of the industry's hiring practices. I'd even say that at least some industries ought to have entirely public-certified hiring norms for certain roles. Maybe that means setting up a standards board or union-linked oversight commission for it.
All the union shops I've heard of have at least a three month probationary period, possibly six, during which you're not part of the union and don't get any special job protections.
Also there are a ton of positions in various fields for which the organization already has someone in mind but still has to advertise the position and do interviews for various legal reasons.
While I'm not particularly pro union, it is not out of the question that a union representing existing employees might bargain for a requirement that all future candidates be union members