I concur with jamesl - U-2(g) is ONLY applicable if there are no rules available in Division 1. In your example where there are rules for dished heads in UG-34, then you are required to follow those rules, in order to call it a Division 1 vessel (and apply the U-stamp).
On the other hand, if the geometry of your head were sufficiently unique that you could argue that it doesn't fall under any of the mandatory rules, then you could apply U-2(g). Please note, however, that validating something to Division 2, Part 5 is much much more complex than comparing some stress to an allowable. I would argue that you have to satisfy ALL of the requirements of Part 5 (all of the failure modes) in their entirety. I appreciate that this sounds like overkill, but the Code authors, in generating the rules that exist in Division 1 (and the design-by-rules in Division 2, Part 4) have considered all of these failure modes (and then some) in the development of the rules. Furthermore, a thorough understanding of all of these failure modes should be a prerequisite for applying Part 5.
<gets off his soap-box>