Spence, my thoughts are that the Atmos design is best implemented by directly downward firing speakers in the ceiling. The alternative of using vertically upward firing speakers(which some makers will supply)on or near the mains may give a reasonable effect which would be influenced strongly by the nature of the ceiling construction. As Dr. Toole has pointed out with respect to the importance of having first reflections remain as similar as possible to the direct sound(suggesting a hard flat surface would be best at the relevant reflection points), so a hard reflective ceiling would appear to be desirable from the standpoint of the Atmos effects.
The acoustic tiles in the ceiling would both reduce the level of the output from the Atmos channels(the amount dependent on the NRC, noise reduction co-efficient, of the tiles in question)and would alter the relative frequency content of the reflections. So, it wouldn't appear to be a very favorable surface for supporting the Atmos effects.
Having said all that, what you propose would have little or no expense and little complexity in making the installation. Setting the M22s at an angle to fire almost vertically at the ceiling(not simply relying on the angled side surface)can be tried to see if even the less than optimal result is still worthwhile in your view.
