The short answer to your question is, "No." Iluvatar split the physical world into two parts and removed Aman from "the Circles of the World". Aman did not become part of the Unseen (if it had, it would have remained in the Circles of the World). Here is how "Akallabeth" describes it:

  "Then Manwe upon the Mountain called upon Iluvatar, and for
  that time the Valar laid down their government of Arda.  But
  Iluvatar showed forth his power, and he changed the fashion
  of the world; and a great chasm opened in the sea between
  Numenor and the Deathless Lands, and the waters flowed down
  into it, and the noise and smoke of the cataracts went up
  to heaven, and the world was shaken.  And all the fleets of
  the Numenoreans were drawn down into the abyss, and they
  were drowned and swallowed up for ever.  But Ar-Pharazon
  the King and the mortal warriors that had set foot upon the
  land of Aman were buried under falling hills: there it is
  said that they lie imprisoned in the Caves of the Forgotten,
  until the Last Battle and the Day of Doom.

  "But the land of Aman and Eressea of the Eldar were taken away
  and removed beyond the reach of Men for ever.  And Andor, the
  Land of Gift, Numenor of the Kings, Elenna of the Star of
  Earendil, was utterly destroyed.  For it was nigh to the east
  of the great rift, and its foundations were overturned, and it
  fell and went down into darkness, and is no more.  And there
  is not now upon Earth any place abiding where the memory of a
  time without evil is preserved.  For Iluvatar cast back the
  Great Seas west of Middle-earth, and the Empty Lands east of
  it, and new lands and new seas were made; and the world was
  diminished, for Valinor and Eressea were taken from it into
  the realm of hidden things."

Nowhere else does Tolkien speak of "the realm of hidden things", but the physical removal of these lands from the world cannot be doubted because of the actual description of the destruction. The "realm of hidden things" may be as close to the "separate reality" Martin is seeking for as Tolkien gets, but there is nothing to connect it with the Unseen, which remains a part of Middle-earth (the Nazgul remain active in Middle-earth, and soon afterward Isildur curses the Men of Dunharrow so they die off and their wraiths remain in the world).

Note that the Ringwraiths and the Men of Dunharrow (not to mention Bilbo, Frodo, and Sam when they wear the One Ring) do not leave Middle-earth -- they do not go to Aman, or Eressea. Wherever Iluvatar put those lands, Sauron's wraiths and Unseen things did not exist there. They existed in Middle-earth.

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Parma Endorion: Essays on Middle-earth, Revised Edition