I am pretty sure that at least all Ostheer vehicles can be hulled down.
IMO Relic created the commander's abilities without even knowing which one will be in which commander in the final game so they probably just made a function that hulls down the area around the targeted object.
For example Soviets had a commander with ISU + IS2 + Bombing run in the Alpha (I think it is also posted in this thread).
I am actually a bit irritated why Allied vehicles can't be hulled down. I'm gonna test that one later.
Agreed, I'd love to have hull down back for the Brits.