The Tomahawks picked players internationals in 2010 that hadn't played any League before with the AMNRL.
SIONE 'PILA' TAUFA (Maui, HA)
LAFAELE 'VAKA' MANUPUNA (Maui, HA)
Are two that I could find without issues.
You can't compare the registration requirements of the AMNRL with that in Australia. Rugby League has quite stringent registration processes within Australia at all levels of the game, especially verses the likes of say Touch Footy or Indoor Soccer, where people often do just 'rock up' for a match.
Niu can pick whoever he wants for the World Cup qualifiers so long as they qualify for the states under the resident or grandparent rules, and that he satisfies the minimum number of domestic players within the squad.
This means he can pick players from the USARL, just the same way as the PNGRFL has picked from leagues within PNG which have no affiliation with the PNGRFL.
Honestly considering this qualification process is the best possible opportunity for the USA to make a League World Cup, Niu would be silly not to pick the strongest possible side ever. Remember there is no guarantee that such a straight forward qualification path will exist for future tournaments. The 2017 tournament might go back to just 10 or 12 teams, or by 2017 suddenly the likes of Malta, Canada, Germany, Greece, Saudi Arabia plus more are in the mix, and the qualification process becomes more like the 2008 RLWC which was always going to be hard for the States.