It might be strange to call Ken Wilber a rationalist, in view of the intrinsic monism and opposition to gross reductionism that is at the heart of everything he writes. Moreover, the whole thrust of his "AQAL" philosophy is to provide a single framework by which all phenomena and all elements and dimensions of human consciousness can be incorporated. And in this he provides a much more successful, and more sophisticated, model then that of similar but simpler predecessors, such as Haskell's Unified Science (which Wilber himself does not seem to be aware of) and Arthur M. Young's Reflexive Universe; even if both of those cosmologie scontain elements and insights that Wilber's does not.
But I am not referring here to the state of transcendental unity (which I fully acknowledge as transrational), or to the grand ideal of integral philosophy, as a system that integrates an dincludes everything (or should), but rather to the fact that his integral philosophical explanation and classification of human knowledge - his AQAL paradigm - is, despite its great detail and sophistication, still presents things in a "biased" manner (which reflects on his entire methodology). And it is from that point of view, this particular bias, that Wilber is indeed a rationalist.
I would humbly suggest that Wilber's Integral AQAL Philosophy is rationalistic in the following reasons:
Anti-ecospiritual. The rejection of neo-pagan, deep ecology, eco-feminism, and nature mysticism (beginning with Sex, Ecology, Spirituality, the book that launched and established his AQAL (integral) phase) which are all seen as regressive and backward (pre-rational)
Anti-occult. The interpretation of magic as likewise regressive and infantile thinking. This would of course be news to a theurgist like Iamblichus, as well as as well as to a modern Hermeticist. In Wilber's writings magic is consistently presented (from The Atman Project onwards) as a state of pre-rational delusional wish-fullfiment. So, no conception of occultism.
Little or no Fine Arts. It is obviously not possible for one man to know something about everything. But the choice of what one chooses to study and not study does reveal on'es interests and inclinations. This is not a judgment, just an observation. So while Ken is heavily into Deveolpemtal Psychology, Mahayana, Postmodern Criticism, etc, he does not seem to be versed in, structural linguistics, Byzantine art, history and its intepreters from St.Augustine and Gioacchino da Fiore to Hegel, Marx, Spengler or Toynbee, Islamic philosophy, discarded theories like Western alchemy, or the birth of modern science in the 15th and 16th centuries
Anti-Romanticism. Ken rejects his earliest Wilber-I Spectrum of Consciousness phase as "romantic" (or "Romantic-Jungian"). One of Wilber's harsh critics, professor of philosophy Christian de Quincey (of the Institute of Noetic Sciences; and Dept. of Holistic Studies, John F. Kennedy University) suggests that "For Wilber, feelings are a lure for "regressive" Romantics who hark back to some mythical golden age, or for narcissistic individuals (typified by the Boomer generation) who prefer the easy, ego-inflation and self-absorption of self-indulgent feelings to the hard road of mature rationality and "transrational" spiritual practice." [ The Promise of Integralism - A Critical Appreciation of Ken Wilber's Integral Psychology] [Wilber's reply Do Critics Misrepresent My Position? - A Test Case from a Recent Academic Journal; de Quincey's counter-reply Critics Do. Critics Don't. - A Response to Ken Wilber ]
"System A" Paradigm. A hierarchical, linear, restrictive conception of the progression or evolution of consciousness; these are all characteristics of what Stan Gooch would call "System A" (masculine rational "day" mind or "ego"), as opposed to the egalitarian, non-linear, mythic "System B"
"Rational" Mysticism. Preference for "rational" secular-western-friendly mystical paths like Advaita Vedanta, Zen and Tibetan Mahamudra, over more controversial (and so irrational) approaches like Luria, Blavatsky, Seth, Shaiva Siddhanta, etc
Pigeonholing. A tendency to very formal classification in all things (in the manner of Aristotle or Hegel), a pigeonholing of all knowledge according to a few basic principles.
Of course, this does not mean that Wilber is purely rationalistic. His mysticism, as mentioned, is transrational (even if it gets there through a very rational route); his holism implies a non-rationalistic conception. But it would seem that overall, rationalism outweighs non-rationalism. And while there is normally nothing wrong with this, indeed rationalism is essential to science (ironically one of the places where Wilber doesn't apply it - see his "irrational" preference for creationist reasoning), if one is to construct a true Theory of Everything, one should be able to equally balance both rational and non-rational.