Exactly right. I'm doing this on a daily basis. Grey resin works great as the mold for silicone parts, and no release is even needed.
Here's the mold injector press I' built and use:
You can see a small mold in position. I clamp it down with a 'Y' bar clamp (you can see half of it in the pic with the red rubber tip).
Here's the reservoir showing the injector tip:
Here's a shot of the mold used in the picture above just after injecting. This mold was actually printed on a 3DSystems printer - the cost of this mold is what made me buy the Form1+