While the glass mount's performance may leave something to be desired (though it may be good enough for his needs), one place to check might be EZ-Pass's (PA/DE/NY/NJ/WV/MA (as Fastlane) RF toll payment transducers) web site for the list of vehicles w/ RF opaque windshields that require external license plate mounted x-ducers.
If the front windshield's not on the list, than the other windows are likely fine too, if it is, the other windows may/may not be okay. Also a dealer or a cell phone company (Verizon/Sprint/etc. may be able to tell you if it's okay for a cellular antenna.
I have seen people use an L bracket screwed to the fender at the hood edge so the screws were not really visible w/ the hood shut, but a hood lip or a rear hatch lip or a luggage rack mount may be other options if you do not want to punch the roof, or have a retracting sun roof or don't want to permanently interfere w/ the roof rack, or whatever.