Kevn- I really like your idea of variable freq and amplitude control for the vibratory process. Especially if using any form of vacuum process during vibration, being able to provide a very carefully controlled level of vibration would be excellent and might allow maximising the benefits of both processes while hopefully minimising the problems created by combining them.
Implementing it with large molds that will weigh hundreds of kg may be a problem though...
Are you sure about this? Assuming there is a solid top plate with weights on top, and the silicone membrane is between the plate and the sludge, surely the compressive force is a function of total weight vs surface area, the same as before?
A metal top plate with a thin silicone sheet glued to it sounds like a good working solution to me.