Marinating lamb that is frozen?

prob too late but i am doubting that a butterflied leg will defrost in just one day, in a marinade or not. In addition, i think pittsburgh is right on the money.

Well, as the lamb thaws, it will release water and this will impact your marinade. How much time do you have? Best is always to thaw in the fridge, food safety issues, yes, but also because it is gentle on the connective tissues and fibers that have been disturbed by freezing to begin. Second best (and pretty quick) is under running water--a boned leg should thaw pretty quickly this way. Ultimately, you want to thaw it and then marinade for 8+ hours to allow the seasonings (I'm assuming there are some) and acids from the yoghurt to do its work. If you are pressed for time, absolutely go with boulangere's suggestion.

It may sound counterintuitive, but frozen food in general will thaw more quickly in a cold liquid (water is usually the one of choice). Warm liquids should never be used, nor should frozen food ever be thawed at room temperature. I suspect you'll have excellent luck and good results marinading your lovely lamb in its yogurt marinade in the refrigerator. There was a thread about the safest way to thaw meat not too long ago here: http://www.food52.com/hotline/13457-what-s-the-safest-way-to-quickly-thaw-out-chicken
The moral of the story is that your lamb will thaw more quickly in the refrigerator in your marinade than if it were simply placed in the fridge by itself. Your marinade sounds heavenly, by the way.