Yes, small bribes can actually be more effective. If you offered £5 for the first person who acted first, I think that would work as a disincentive - people would be embarassed, not want others to think they needed it (or maybe that's just us being Brits?). Whereas a Jaffa cake shows esteem, and lets someone pretend they're just doing it to get the cake.

People enjoy play and small ritual. Making light of something lowers the barriers, doesn't ring the cthulhu-horror-of-being-manipulated bell because it is openly ironic.

{I know "ironic" isn't the right word, but I'll stick with it, because I think people will guess from context what is meant.}