This is a very interesting question. It might even be worth submitting to Modern Steel Construction's Steel Interchange. Per the AISC 360-10 user note at the beginning of Chapter F, you are allowed to use the interaction equations in Sections H1 for beams in biaxial flexure without axial load.
Interestingly, Salmon & Johnson's Steel Structures Design and Behavior 5th Edition, Section 7.11 suggests that the beam-column interaction equation is unconservative without axial load. The author suggests not relying on any plastic capacity. That is, for the yielding limit state, Mnx = FySx and Mny = FySy.
Based on my Google searches, it appears that Australian standard AS4100 Section 8.3.4 uses this approach.