Looked into this a year or two ago at my prior gig. The ASA is unambiguous in that an SRNA is not, as they put it, a "qualified anesthesia personnel," simply meaning they are not allowed to be left in a room alone, more akin to a medical student. Furthermore, they state that a physician supervising an SRNA does not constitute a valid anesthesia care team. Sure, the ASA statement is not law, but if billing medical direction, you would potentially be exposed to liability for medicare fraud. From a medico-legal standpoint, speaking as someone who has consulted on both sides, should an event occur and a suit were brought, you would be eviscerated.
That having been said, we talked to people from name-brand places across the country, and the practice is common.