Hmm, by the description of Ilmaren, then, "flesh unaided" should probably not be able to survive there, either, given that the uttermost height of Taniquetil appears to pierce the habitable regions of the atmosphere. And yet, if I'm not mistaken, weren't some of the Elves invited to Ilmaren (most notably the instance when Manwe was trying to get Feanor to reconcile with his half-brothers)? If they could survive then, and if Earendil can survive his nightly passages in Vingilot, then I think we can find the fingers of the Valar at work, protecting the various incarnates in areas where they should not be able to survive. We are shown the moment of Frodo's arrival in Aman, and he does not appear to be a spirit; indeed from the description in LotR, he doesn't even appear to have noticed that the ship ever left water. We also know from a variety of places that the permission given for the Hobbits to reach the West came from the Valar through their remaining representative in ME, Gandalf. So it seems to me that we have two versions of such a journey being posited: one in the legends of Men, who appear to believe that the ship leaves the water and passes on into the air and the void, where the passengers must be protected by some greater power to survive until they arrive in the detached Aman; and another in which it seems that the ship simply passes through a "curtain" separating one world from the other, and the passengers never notice the moment of transit as anything more (this being more or less what is described at the end of LotR). The latter version may not be quite accurate either, since it is in Hobbit lore, and appears to be based on knowledge of Frodo's dream/vision in the house of Tom Bombadil; I would presume that the very last parts of the Book were written by Sam and perhaps Merry and Pippin, based on what Frodo revealed in what he himself wrote before he departed Middle-earth. The one thing that does appear to be rather consistent is that permission from the Valar is required for the ship to properly enter the Straight Road (that permission might be in the form of what Tolkien described as "hallowing," the process of which he never describes, though I imagine it is a ritual only the Elven shipbuilders know); lacking it, it would merely stay on the seas of earth.
What would happen to a stowaway... I suppose it would depend on whether or not the Valar were aware of his or her presence onboard. I suspect they would be; the fiasco of Numenor would probably have them on guard about such things. I would imagine such a ship would fail to enter the Straight Road, the crew and passengers would begin to wonder what was up, and a search would be made until the culprit was found and returned to ME. That presumes rather a lot, I suppose.
