ASME does not segregate processes by pass, or variables by coupon. In other words, you run a coupon with say GTAW or SMAW. ASME does not distinguish variables that would prevent using the SMAW as a root pass. And backing gas is a non essential variable.
However, once again, this does not relieve the engineer from the responsibility of sound engineering judgment. In this case, being SS you would most likely have to add a metal backing such as a chill ring to support the SMAW process(backing also being anon essential variable). And there is also the issue of the HAZ to consider, and this is the critical point for SS. So therefore, as I would not recommend using the SMAW process for the root pass for this reason, there is nothing in ASME that prevents it.