Short answer: depends on the glass, but generally not a problem.

Any glass you put in front of a camera (any camera) is going to reduce the light that reaches the sensor.  Most people designing optics systems add AR (anti-reflective) coatings to reduce this to the single percentages. 

In the case of the PerC camera you will also need to make sure the glass you use does not filter out the IR light that is passing out and back again.  Generally speaking most glass you find should not have a problem (even without an AR coating), but you will need to just test it out and double check.  I know of one company using the PerC camera behind glass for a holographic display system without any problems, and customers have used our older generation cameras (based on our same sensor technology) for digital signage displays behind glass for years.  If you use tinted glass you need to make extra sure the tint does not filter out the IR frequency.