You can certainly wait to register an LO until they reach the threshold; BUT you must a have a tracking mechanism in place to 1) demonstrate that an LO qualifies for the de minimis exception and 2) determine when the LOs should be registered.
It would be easier to just register one LO and direct all covered loan applications through that one person.
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I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.