AussieNick,
Your reference ENR 1.1 - 37 is valid for enroute navigation. A DGA is a considered to be terminal procedure (ie navigation in the terminal area) and so this reference is not applicable.
As the approach has been designed and cleared to either the NDB or VOR, then you are perfectly ok to use either, whether you have them both or not. I know I would use the VOR, but the point I am trying to make is that you don't have to in accordance with ENR1.1 -37
Just my 2 cents from a procedure design point of view
Alpha