If this block heater is going into the side of the block then it seals on its own with a small tamping type of Hammer in my mind. I don't follow where the need of any Silicone would be required. Just can't understand with all the pressure from the heating system how it would keep a Frost plug in with Silicone. 
Barrie I think your thinking 60s stuff.

The block heater that will be going into this Mazda you do not tap into place like a frost plug. (see pics) The silcone doesn't hold the block heater in barrie as you know, it just assures that it doesn't leak because not all block heaters are going to fit a used motor perfectly. Don't compare the fit of an aftermarket block heater to that of a factory frost plug installed when the motor was assembled. I use silcone when installing water pumps. I'm not doing the job over again because of a fault in the casing created when the original fitting was removed.
As you can see in the pic the block heater comes with a rubber o ring. Silicone just adds another layer of leak protection to that. There is no guarantee that the o ring will be an exact fit.
In the alternative I agree with these guys: This is a passage from Car Talk (a US syndicated radio car tech show heard across the country).
Ray: The freeze plug replacement heaters work pretty well, and in fact, that's what Ford gives you if you buy one from them. The heating element actually replaces a freeze plug in the engine block and heats the coolant from down there. My biggest concern about freeze plug heaters is that they can leak after installation (we've even seen a few blow out under extreme pressure). And if you don't get under the car and check it regularly, you might not know it was leaking for many moons.
Tom: So for that reason, we're partial to coolant heaters that get installed in one of the two radiator hoses. They're easy for your mechanic to install, the installation almost never fails, and they do a good job.
Ray: You need one that fits the diameter of your particular coolant hose. The heater itself costs about 40 bucks, and you should be able to get one at any good auto parts store. Stay warm, Sidney.
On top of that the poster will need to decide which frost plug to knock out. What if it is the wrong one

Why bother with all that?