I would suggest that this question is not one to be answered by some anonymous clown (i.e. someone like me) on a free internet forum where the advice you get is worth every Euro you pay for it, but rather something which should be a matter of agreement between you, the designer, and the owner/user's engineer.
B16.34 allows other materials than those directly listed provided they are listed in the boiler pressure vessel code with allowable stress versus temp. values. If so, then you establish the pressure - temp. rating per the method defined in B16.34. If the material is not in B16.34 or the boiler and pressure vessel code, it cannot be a 16.34 valve.