EVER mindful of its role as the opium of the teeny masses, MTV is to hold an open forum in Hong Kong Park with 50 young Hong Kongers on the vexing topic of AIDS. And they'll get plenty of novel advice. For among the experts on the panel is Mechai Viravaidya, Thailand's ''Mr Condom'', who frequently chooses to arouse public awareness on safe sex among the young in his own country by organising events with droll titles like ''Cops and Rubbers''. At the MTV recording of the forum next Saturday, photographers - given the occasion - are being asked to be on their best behaviour. For the fax details say: ''Photographers welcome. No flash please.''