I wouldn't mess with hotels in Asia about the number of people in the room -- if they find out, unless your very important to Hyatt, they're likely to charge you. Century Hyatt is also franchise so not sure what their policies would be...
You might be interested to check out the following post about what a Starwood hotel in Spain did to someone with an "extra guest" which wasn't even an extra guest!
http://www.flyertalk.com/forum/Forum74/HTML/004698.html
I'm with letiole on this one -- spend the extra Y3000, tell the truth, and get an extra bed if you want it.