A few people in my trials club here in CQ have got QLD rec rego on their (non-ADR) modern trials bikes in case they were stopped and questioned by police or rangers in one of our local State Forests, but they have not been stopped yet. I remember they told me it was quite comical when they were doing the paperwork with the Transport Dept clerk when they told them where to attach the plate. They protested a bit and then googled up a photo of their trials bike on the Transport Dept computor to show that there was actually no suitable place to attach it in the way the rules require.
