Apparently kiwi fruit can be used, as it has an enzyme which works on the meat to tenderise it. According to a couple of sites I found you puree kiwi fruit in a food processor or blender (or I suppose you could mash well with a fork if you don't have one of those gadgets) and smear it on both sides of the meat for three hours or so. When ready to grill the meat just wipe off the fruit first and it doesn't affect the meaty flavour.

Let us know what it's like!

Cheers
JennieG