When I think of metaphysical writing, I think of the complex essays and expostulations of kabbalistic authors. The best of these are elaborate and clear mental constructs that can be difficult to understand, not because they are obscure, but because they are so carefully precise in their detail. I admire these writings, but I read them infrequently, because they take a lot of effort.
The same is true of many writings on Buddhism. The Tibetans have developed a metaphysical system which is vast, and like kabbalah detailed.
If I am going to read metaphysical writings, I much prefer to go to Rumi, or Hafiz, both Persian poets. Hilmi Yavuz is another one I love, though, unfortunately, I know of only one book of his in English, "Season of the Word."
At some time in the recent past I asked myself the question, "can metaphysical haiku be written?" and the answer, of course, is yes. But due to the nature of the haiku form, metaphysical haiku will be the exact opposite of kabbalistic and buddhistic discourse. A haikuist cannot embroider a thousand details with a tiny needle, or elaborate on the fine points of various types of energies or levels of existence
But what he or she can do is shine a ray of light into the subconscious of the reader -- sound a spiritual chord of resonance, so that the reader feels the metaphysical truth, even though they may not be able to clearly define it in conscious, intellectual terms.
I have come to believe that all poetry is metaphysical truth -- it's just that a philosopher prefers the term "metaphysical truth," and the poet prefers the term "poetry."
Just as the metaphysician says that everything has its source in love, the poet knows that all emotions are expressions of love, and all negative emotions the frustrations of love, and that poetry, to be true poetry, must have emotion, that is, love.
One of the most interesting things for me about writing haiku is that as I do it I feel I am actually two people engaged in the act: one is a person who is consciously and intellectually making word choices and trying to be objective and critical about what he's doing, and the other person is an inner voice that occasionally just blurts something out. Of the two, the blurter is the best poet, though the intellectual/critical one can play a positive role when he comes in to support or polish the blurtings.