E-marked or not I'd be very wary about attaching any load to the glass of an existing standard mirror. For a start, it applies an additional load to the mirror adjustment mechanism - something it was never designed for. Secondly, the standard mirror glass is only stuck in place. Who knows how well this bond will hold up over time if mirror extensions are attached directly to the glass. Thirdly, many mirror glasses also have heating elements in them so if they break due to an unfortunate knock on the 'Suck-it-and-See' mirror, you've got an expensive repair on your hands.
I'm also not sure whether the law does not require unimpaired field of view through the standard mirror, even when additional mirror extensions are fitted.