I made mashed banana/cocoa icing once and it was hideous and slimy. And I love banana. Can you do melted dark choc, with a little honey? Or, if its a muffin, try a sugar/cinnamon topping before you bake. You'd need less than half a teaspoon per muffin.

If you wanted to decorate after, I'd go with a cream cheese type thing and then dip in coconut, dried fruit etc

Hard isn't it?

Oh, and I just thought....can you make the muffins sugar free so you can use the extra sugar rations in the icing??!!