What you need to do is work in the expanded view, and then
orient your WCS to the plane that you want your dimension on.
You will dimension it in that expanded view, and the
dimension will be placed on the plane of the WCS.
The issue comes up that the dimension will need to be scaled
because of the drawing view scale (unless it is full scale) -
place the dimension down and then scale it using "transform".
Of course if you have PMI you could create those 'Isometric Dimensions' in a model (PMI) view
and then simple add that view to the Drawing.