Yes of course dip molding silicone over a bare PCB can work, similar is frequently done on some assemblies. PCB’s are often conformal coated, usually a spray, sometimes a dip, for extreme insulating purpose, emergency certification approval, etc.
However, a general rule is that PCB’s are never structural nor should be directly handled. Solder joints are structurally weak. Flash drive insertion / extraction hand grip force will quickly damage the PCB components or flex the thin glass epoxy board and crack tracks or solder joints. Also, you run the risk of directly introducing electrostatic shock to a sensitive component if the silicone is thin or wears off. It would be safer to do a clear acrylic cast, however without looking someone probably already has…