[Beanamir of Gondor]

Yes, Frodo-Sauron-Grishnakh is a tie with Beren-Morgoth-Carcaroth. Anyway, since you already know the name itself, here's my reasoning behind it:

Frodo entered Mordor dressed as an Orc: Grishnakh was a notable Orc who served Sauron, Lord of Mordor. (You're right in saying that Shagrat would have done better as an example, because he was at the gates of Mordor when Frodo entered.)
In any case, Beren entered Angband dressed as a Werewolf: Carcaroth was a notable Werewolf who served Morgoth, Lord of Angband.

So there it is. Thread's yours, Legate.