I have not personally worked with nitrogen purging of 300 series stainless steel, but have heard about it over the years. AWS C5.5/C5.5M:2003, Recommended Practices for Gas Tungsten Arc Welding, states that nitrogen is widely used in Europe as a purge gas, and that it is used for austenitic stainless steel, copper, nickel alloys, carbon and low alloy steels, but it should not be used with reactive metals.
I think there could be a couple of concerns with purging austenitic SS.
As I understand it, Nitrogen is a diatomic (2 atoms) molecule under "normal" conditions, and the N2 molecule it is basically inert. But, N2 breaks down to single atoms in the welding arc and becomes very reactive with molten and hot steel, so Nitrogen cannot be used as an arc shielding gas. But, inside pipe or tubing with closed root weld joints, N2 can be used as a purge gas since it is not exposed to the welding arc.
Nitrogen is a strong "austenite former", so if nitrogen is "picked-up" by the weld metal, it will reduce the ferrite content. Somewhere around 3 to 10% ferrite is generally recommended for prevention of hot cracking.