Without actually being able to look inside the case, I'd recommend against it.

Even assuming that you could drill through and not hit one of the active elements, any metal, conductive or not, ferrous or not, that close to the radiating elements will absolutely distort the pattern.

There is also a chance of increasing interference inbound or outbound (there are a lot of variables ... size of the screws, composition of the screws, proximity to the elements ...).

You may be making enough of an alteration to the antenna that it no longer is acceptable as similar to the product licensed by Cisco to sell in your city/county/state/country; meaning that o remain legal, you may have to have it re-certified.

The antenna comes with clips to attach it to (at least) two types of drop ceiling. Why not just use the clips?

Good Luck

Scott