Jennifer,
I would not recommend using silicone to hold the sword in place! There are many other ways to make brackets that will hold the weight and not allow it to fall forward.
One technique is to incorporate a piece of coroplast behind the matboard. I would go to Home Depot or Lowes or Home Depot and buy a piece of 3/16" brass rod. You can cut and bend the rod to whatever shape you choose to hold the sword around the hilt area. I would put a "U" shaped saddle in 2 rods that would be mounted on either side of the hilt and just deep enough to seat the sword evenly in the saddles. On the other end of the hanger I would bend about 2" of brass into a right angle and mark the 2 places on the matboard where the sword will hang.
Now the coroplast comes into play. Cut it to size with the flutes of the coroplast running vertically. You may want to adhere the coroplast onto the matboard for added strength. Punch 2 holes through the mat and the coroplast where you marked them. Now take an Exacto knife and cut out each flute in the coroplast to form a trough from the hole downward for a distance to equal the length of the right angle bend you put in the brass rod. Push each brass mount through the corresponding hole and allow that "leg" that you bent to lay down in the flute of the coroplast that you cut open. Now fill that flute with hot melt glue or epoxy glue and also glue down the coroplast flap that you had when you cut the flute open.
After both hangers are glued in place you have the option of gluing this package to the ply board backing for even more added strength and I am quite positive that this will hold your sword.
I am building the same kind of mounts right now to mount an autographed Fender solid body guitar in a 2'x4' shadowbox that will have a heavy Roma moulding hinged to the front of the box so the owner can open it to remove the guitar.
If you want to pad those brass mounts to minimize any chance of scratching the object, you can use heat shrink tubing available at Radio Shack or ask about a plastic product at Lowes called "Plasti-Coat" or "Plasti-Dip", it comes in a tall can (like a tennis ball can) and is available in black, red, and maybe blue. You can dip the brass hangers in this stuff and, when it dries, you have a nice plastic coating on the hanger that should protect anything that is hung on it. This stuff is used for replacing the plastic grips on hand tools when they wear out or come off and is really neat stuff.
Framerguy