I think this is counter to the Retool business model. They need to get a cut of every person that makes use of Retool. The also do not allow a third part authentication as an end-run around this.
However, similar method that does not need authentication could work using a public facing app. A "user" has a long, unique ID that is passed in through the querystring. Anyone on the wire could read the URL so I would not call this secure, but would be useful for certain situations where security is not an issue. I have a client with low security requirements who needs to service many people outside their office for but a $50/month license for every one of those is not workable for their business model. I will be trying that model out on them, which I believe is the intent of the public app feature.
I have a somewhat related discussion on this forum category (Using Retool as a SaaS platform for verticals - #3 by bradlymathews) about creating a vertical SaaS application.
I would love to hear other people's ideas and viewpoints around Retool SaaS potential.