[Hitton]

>because who would trust an SSO created by one guy in his basement

You don't have to tell everyone that you are one person company operating from basement. Why not use phrases like "our company", "our clients", have different email addresses like support@, sales@, hr@ subtly insinuating there are more of you, use picture from photobank when listing address of your company's headquarters(basement) etc.. You won't be first nor last person doing this.

And Open Source/Proprietary doesn't need to be either/or problem. You can dual license it as AGPL/Proprietary and make contributors sign CLA.