Yep, this works. If you can get hold of a Verizon iPad (or IMEI/MEID), you can activate the SIM card using that and then pop it over to the non-Verizon iPad and it should work.
Neither Verizon nor Sprint really has to keep track of each individual iPad sold. Apple configures the carrier and installs the SIM cards in the factory so all Verizon or Sprint needs to do is get the list of IMEI/MEID from Apple. Heck, I'm guessing they don't even need a database of each individual IMEI and just go by the IMEI/MEID range. Given Verizon already keeps a database of the ESN/MEID of who knows how many phones in their system, I don't think adding a few hundred thousand more MEID's for iPad's would be too much of a burden.
Yeah. I actually have a Verizon rmini and a Verizon sim. But I've got a att unlimited data iPad plan on it.