To take this more seriously than is necessary, I am reasonably sure (although others may know better) that the term 'Maiar' was not even used by Professor Tolkien to refer to the lesser Ainur until after the composition of
The Lord of the Rings. Certainly Gandalf still refers to "Fionwė son of Manwė" in drafts of the confrontation with the Balrog if I recall correctly, which were composed after the Bombadil sections were written (and I believe they were not substantially altered afterwards). In fact I have a rather firm impression that the very concept of the 'Maiar' as we now understand it was not solidified by that point, where there were still 'children of the Valar' and 'folk of the Valar'.
Christopher Tolkien himself observes that the 1958
Valaquenta is "probably where the word
Maiar first arose." (
Morgoth's Ring) Anyone performing more than a most cursory research into Professor Tolkien's process of composition (for a publication, for instance) would be able to discern this information.
One also cannot help but think that if there was some groundbreaking secret about Bombadil's identity it would not be the same trite, cliché line of speculation which has been proposed (and to my satisfaction at least, refuted) for years and years: "Bombadil is a Maia." How shocking.
