My concern with that would be access to the mirror screws if needed. While not impossible, you would have to dig out the RTV. I have already had the passenger side one completely off the truck to install a "trucker-style" Sirius mirror-mount Sat radio antenna.
When I took my first short trip in my unit I noticed quite a bit of mirror "shake" and so I investigated the issue when I got back. I was completely missing one screw in passenger side top bracket (meaning only one screw holding it!). The threads were either messed up or never cut so I even had to run a tap through it when I received the new screw from Freightliner. Probably why it got skipped at the factory, who would know! The screws do come with a bit of thread locking compound on them, but even with that on the original screws probably half of the them (both mirrors) were somewhat loose! After tightening all of them the "shake" problem is better, but not completely gone. Acceptable for a diesel truck in my opinion.
My plan is to check the screws for tightness periodically, would be harder if holes filled with RTV.