Peter, let me lay it out straight for you.
Football pitch rental is as valuable a commodity as iodised salt in these parts. It is INCREDIBLY difficult to book a pitch in a legitimate way. If you don't believe me, go to LSCD Leisure Link booking and try it out yourself.
There's a Chinese secondary (black) market which isn't really available to foreigners. Basically, people use their inside connections to book all the pitches and then they charge 3/4/5 times as much as they paid to re-sell the pitch.
If you do miraculously find a pitch though (sometimes a slot or two falls between the hawkers' greasy fingers), I play for a seven a side team and we would be happy to give you boys a game.
Edit: and to add, yes, your friend is right. You do need a HKID card to book pitches. I don't know of any bookable pitches outside the Leisure Link system.