You can indeed use your surprise round partial action to ready a partial action. Ready is allowed as long as you are not outside the combat round-about.
I think you may ready the "start full-round action" partial action, but in that case you're not completing the full-round action in the surprise round but instead it has its effects in the following round.
So for example you could ready a full-round casting spell or in your case the full-round throwing of a 2-handed weapon; you'll be anyway completing the attack only in the next round, when you'll have a standard action left (I mean, without a mea), and you'll have a lowered initiative.
I am not 100% sure that it works when the full-round action you are "starting" is an attack, such as here... what happens if your target (if he's got to act in the surprise round, or if he comes before you in the first regular round) moves out of the way, maybe on the other side of the room?
I'd rule that you still release your attack in the direction you start throwing, it doesn't seem to me you may easily re-aim such a bulky throw.