What those making the rules need to do is define the word "mounted".
I feel sure that what is intended is to have any ballast ahead of the rear axle line, apart for the types of vehicles given exception from the rule. If that is the case, why not say all ballast must be located between the front and rear axle centrelines or some such?
I would have thought you can 'attach', 'secure', 'fix', 'bolt on' the additional weight material known and defined as "ballast" but to me you would not necessarily ever "mount ballast" to any vehicle.
Hey, but what do I know when my English differs from most of you folk!