I asked about this "Nationals" and "Worlds" usage thing on the Sailwave User Group. Apparently you can have a Nationals - there is no problem with that (need MNA approval?) - it's only "Worlds" that you need ISAF approval for if you're not an ISAF class. The requirements are in regulation 18:-

http://www.sailing.org/regulations/2006_partIV.pdf

http://www.sailing.org/regulations

Essentially 25 competitors across 6 countries in 2 continents. If you can make that, you could call the Singapore event a World Championships after seeking approval from ISAF; why would they try and hold a thriving new class back...?