I think comb filtering will come into play as the glass will be very close to your mids, but wether that ends up being a bad thing????

I've heard a few cars that sounded really good with their mids up high on the dash.

To each there own I guess.

 

^ agree

The key is to control the direct signal vs reflected..... if you could get a clean path for the direct (near a 10ms window will be ideal) it'll be fine. there are also ways to reduce comb filtering issues by manipulating the polar response of the drivers.