don`t worry about it at all, I`v done it when the whole thing`s still frozen and it`s turned out perfect, when you`re cooking a roast like that for so long, half an hour makes no real difference :)
in fact sometimes it`s an advantage using frozen meats, as you can drill holes into it and pack in herbs and spices and then plug the holes again with the meat turnings, gives a great effect and flavor throughout.
use a Cordless drill for this though, there`s no risk of electric shock that way.
Katherine Snow. xx