the lord of the rings - Where did J.R.R. Tolkien state that Arwen suggested that Frodo be allowed to sail West?

In The Letters of J.R.R. Tolkien by Humphrey Carpenter (assisted by Christopher Tolkien), footnote 94 states:

What is meant is that it was Arwen who first thought of sending Frodo into the West, and put in a plea for him to Gandalf (direct or through Galadriel, or both), and she used her own renunciation of the right to go West as an argument. Her renunciation and suffering were related to and enmeshed with Frodo's : both were parts of a plan for the regeneration of the state of Men. Her prayer might therefore be specially effective, and her plan have a certain equity of exchange.

I don't remember that being mentioned in The Lord of the Rings, not even in the appendices. Where (in published or posthumous material) did J.R.R. Tolkien make Arwen's role explicit?

Answer

In fact this was mentioned in The Lord of the Rings, though perhaps in not as much detail. This comes after the Ring is destroyed, and everyone is celebrating in Minas Tirith. Arwen and a bunch of other elves have just arrived, and Frodo has gone to talk to Aragorn and Arwen:

The Queen Arwen said: "A gift I will give to you. For I am the daughter of Elrond. I shall not go with him now when he departs to the Havens; for mine is the choice of Luthien, and as she so have I chosen, both the sweet and the bitter. But in my stead you shall go, Ring-bearer, when the time comes, and if you then desire it. If your hurts grieve you still and the memory of your burden is heavy, then you may pass into the West, until all your wounds and weariness are healed."
_{The Lord of the Rings Book 6 Chapter 6: Many Partings}

So when Arwen, like Luthien of old (with whom she was often compared in beauty), chose the mortal life with the man she loved, she managed to arrange for Frodo to take her place if he wanted to.