Mike, the way I read the proposal, it sounds as though they're trying to bring into the testing regime vehicles which are based on type approved, mass production type vehicles; so to take road construction vehicles, a white line marker built on a truck chassis would be testable, but a road planer would be exempt.
I suspect the issue with testing Ferrets and the like, apart from the fact they're classified in an exempt group, might be as much to do with the practicalities of testing as much as whether they can physically be tested - climbing in and out, testers driving them, and so on.