Or, we can use what worked in the past without involving for-profit enterprises: grassroots movements

Easier to align people when you remove the whole troublesome "money" part. Question is how to motivate Americans to work together if not for money?

Who would be purchasing its services?

The only obvious customer would be a union, and they already provide that service themselves.

Unions sometimes hire on third party organizations to help them organize, I don’t think they are specifically specialized against anti-union consulting firms, but I bet that’s part of it.