Mr. Google is your friend.

From the NC Wildlife Resources Commission web page for registration and titling:

1. All sailboats used on public waters must be registered if longer than 14 feet at the load waterline.

2. Rowboats, canoes, kayaks, and rafts moved only by oars, paddles, or the current do not need registration in North Carolina

So as long as it has no sail, your kayak needs no registration. I'm guessing that's what the response about an "instantly insteppable" mast was all about. When you see the water cops, just unstep the mast and turn into a kayak again. Good luck with that :-)

More seriously, how long is your kayak? Remember, the load waterline (LWL) can be significantly shorter than the length overall (LOA). Any boat  15 ft LOA or less, like the Woodducks. is probably close enough that the water cops won't bother to measure unless they're in a really grouchy mood. 

Best bet is to contact them, explain what you have and see what they say. Their contact info (again, from the web page) is:

By telephone at 800-628-3773, Monday-Friday, 8 a.m. - 5 p.m.

Good luck,

Laszlo