A few ideas that come to mind;
how about before you start a bucket of nasty stuff is made up; maybe some beans, mushy peas, gravy, and any number of other slimy things you have in the cupboards. After the game, the loser must put on a swimsuit and be covered in the mess from the bucket. I dont know what your limits are, but the loser could possibly be tied up to make sure they cant escape.
Another idea that comes to mind is for the loser to have to put on a pair of plastic pants. The winner can then fill the pants with whatever they fancy. The loser must then spent a decent amount of time in the pants before taking them off. Again, depending on your limits (and how much mess is in the pants) the loser could be forced to put on their normal clothes and go out in public with the messy pants on beneath their clothes with only the winner and loser knowing what is going on. With the right sized pants, the mess shouldn't leak out as the pants are pretty good.
Are these the kind of thing you had in mind? If you want any more ideas let me know and i will have a think.
Thanks
CZ
p.s thanks for the pics!
p.p.s A link to the plastic pants i mention above;
http://www.ebay.co.uk/itm/ADULT-BABY-PL ... 3ccef03747