Worshiping a disembodied Buddha-nature is the same as the Shingon worship of Dainichi Buddha:

Namu means to worship ["In worshiping all the deities and Buddhas, the word Namu is put ahead of their names." -- Nichiren]. We worship Myoho renge kyo and the Gohonzon. Then let us ask ourselves, what is the nature and reality of the Gohonzon? Nichiren Daishonin teaches the Gohonzon in terms of the Person and in terms of the Law. We know the Law is Myoho renge kyo or Namu Myoho renge kyo [5 or 7 characters]. Who is the “Person” of the Gohonzon? Nichiren specifically teaches that it is Shakyamuni Buddha of the Juryo Chapter of the Lotus Sutra, the Three Bodied Tathagata, one of whose bodies is Shakyamuni Buddha of India. Generally Nichiren Daishonin teaches that the person of the Gohonzon is each one of us who chants Namu Myoho renge kyo and has faith in the Lotus Sutra and Shakyamuni Buddha. Never once have I heard the expression, Namu Buddha-nature nor Namu Buddha-wisdom from either Shakyamuni Buddha nor Nichiren Daishonin. Neither nature nor mind is the entirety of Buddha nor of man. Worshiping a disembodied Buddha-nature is the same as Shingon worship of Dainichi Buddha:

http://en.wikipedia.org/wiki/Dharmak%C4%81ya.