Oh. I was assuming the silicone would go under the gasket and seal against the body, not on the bottom of the windscreen, and thus on the top of the gasket. If under the gasket, a mess on the underside of the leather would not matter. But I see what you mean about sealant on top of the gasket.
Hmmm. Anyway, mucho caution would be needed not to make a mess if silicone is involved.