Menachem Kellner wrote a book on the topic, where he builds on work by Moshe Idel. Kellner takes for granted that Rambam rejected (what was taught as) kabbalah of his day, and his book 'shows' how much of the Rambam's writings were aimed at presenting an alternative to kabbalah. You could read his book as indirectly proving that the Rambam rejected kabbalah, because if he's writing with an anti-kabbalah agenda he obviously rejected it. However, in my mind, Kellner's book still doesn't prove anything conclusively.
All in all, the scholarly consensus is that the Rambam rejected kabbalah, or what he knew of it, and not just because he writes in a teshuvah (Blau ed. I:117) that the kabbalistic book Shiur Komah is heretical. The Rambam's whole outlook doesn't seem to fit with many of the tenets of kabbalah, at least of what we know of it from that time. While you're right that these things were kept under wraps by those who knew them, the Rambam states in the introduction (and throughout) Moreh Nevuchim that in his book he is revealing the secrets of מעשה בראשית and מעשה מרכבה as he understands them, and they certainly don't look like kabbalah.
Of course, this is all kind of tenuous because we can't be sure how to read the kabbalistic works just because we have the texts. Maybe we're reading them incorrectly, as many authentic mekubalim (like R. Yaakov Hillel) of today will tell you that certain terms are codes for certain concepts, and that the mekubalim speak metaphorically, etc... This is especially so because later mekubalim actually did use the Moreh Nevuchim and picked up some of the Rambam's terminology here and there.
(For the sake of completeness, however, I should mention that there are two letters supposedly written by the Rambam which do in fact praise the kabbalah, though scholars have dismissed them as forgeries. One of them is quoted by the Migdal Oz to the first chapter of Hil. Yisodei HaTorah, where the Rambam is said to have discovered kabbalah later in life, and another well known one published in R. Shilat’s edition of the Rambam's letters, pg. 695. See there where he discusses the veracity of the letter)