Must say I've never heard of it done with offal - I'm going to give that ox heart a try! - do you do anything different in the preparation?
You have to remove the thick catilige from the centre then slice it in the same way you would with other cuts marinade leave in fridge overnight or vav pac for a few hours to get the flavour into the meat I was advised to put the meat in at a high tempeture for a couple of minutes then keep on a very low heat untill completely dry
I posted a biltong photo tutorial thread a few years back where I had people say that you can leave the fat on without problem. I've tried - and they were right! For my personal tastes I still don't like too much of it, but I'm less thorough in my fat trimming than I used to be