This is a difficult issue. There are lot of reports that claim that they achieved homogeneous shells and also they can tune the thickness. Many of them lack of reproducibility.
In my experience there are many parameter to take in to account when you try to coat silica with a metal.
We have recently published a paper of the formation of Ag NPs over SiO2. From the point of view of the interaction between the materials.
JOURNAL OF COLLOID AND INTERFACE SCIENCE 428:32–35
Article One-step/one-pot decoration of oxide microparticles with sil...
we have a small sputter equipment with a Gold source that can be used to coat almost every surface at room temperature. I've used it also to coat silica nanoparticles that were deposited on a substrate. Normal PVD should equally work using E-beam or thermal deposition.