Canada is a lot lower key about such things. Most plants are registered through COPF, and that is a self reported royalty system. The fees are low enough that it's not worth the hassle to cheat. (Typically 10 c to 50 c per plant.)
I've never seen an ag inspector other than the county weed inspector. Our county and the one to the south share a horticulture agent. She's a resource, not an enforcer.
How would the agent know if you had another 5000 square ft (or 50,000 square feet) yard somewhere else?
If you buy X registered willows for resale, then, as long as you show less than X in your yard, how do they know you are in violation? Do they go through your books? (In which case I would keep my books on paper using roman numerals just to piss them off.)