I agree that no cases of such surrenders are recorded, but I think they are
implied though rare. Looking further at the passage from Morgoth's Ring I
previously quoted from it says, with my own emphasis;
"That is, that though of necessity, being the fingers of the hand of Morgoth,
they must be fought with the utmost severity, they must not be dealt with in
their own terms of cruelty and treachery. Captives must not be tormented,
not even to discover information for the defence of the homes of Elves and
Men. If any Orcs surrendered and asked for mercy, they must be granted
it, even at a cost. This was the teaching of the Wise, though in the
horror of the War it was not always heeded."
This I think does show, in the context of Orcs, a code for the treatment of
prisoners of war, but that the code was not always followed. This last
implies however that it usually was, which further implies that Orcs did
occasionally surrender, and when they did they were shown
mercy.
