Here is an extract from the rules where I live. Under the seat plate doesn't work for me. Cops would care if they saw it. But more likely it would be an add on fine after getting nicked for something else.
I live with the same constabulary as Gcduc and have the Evotech tail tidy. Motorcycle copper pulled up beside me and said I needed a reflector under the plate but all good otherwise. One came in the kit so I stuck it on when I got home.
He didn't mention the sound of the un-dbkilled Full Termi funnily enough.
AH, good to know. I don't have a reflector but one can be added easy enough. Makes sense that the motorcycle coppers would know the rules. I think the usual patrol would not.
You would feel seriously upset for getting booked for the termi given some of the noise the HD crowd make.