Just wondering if a current or previous client could serve as a reference?  So, for example, you had one provider reference and one friend/buddy who has visited you before and referred a new guy to you, would that serve as a second reference?

The provider has to have a level of trust to be able to do that. You also need to know that you hobby buddy won't do stuff that will piss of the provider (I.e demanding services she does not offer, dick to big, etc). It helps if the provider, you, and your hobby have to same type of kinky interests. She will ask he new client bring your toys lol.

...in addition to a reference from another provider or with other information that I ask for, but definitely not as a stand-alone reference.

...and keep her in the loop about it. That does not, however, mean he would get to remain anonymous and as Erin said, screening would still happen.