Can the word myriad be used in a stand-alone fashion, without a subsequent of prepositional?
He rose to address the myriad, and wept.
The word is a noun, and this usage sounds poetic to me, and I can't think of any reason beyond convention why it must precede an of (especially if there is sufficient context establishing the items being vaguely quantified)
Perhaps switching the article makes it more awkward:
He rose to address a myriad, and wept.
However, qualifying it suddenly brings it around:
He rose to address an assembling myriad, and wept.