Dude thats a tough one...
If you are lucky and know a machinist, or have access to a milling machine...
1.) Bore out a slightly larger hole and tap it to standard size. (Make sure the hole isnt too large or else you wont be able to get a screw that can screw through the mobo.)
2.) Find the screw that fulfills that new standard size hole.
3.) Find springs for the screws
You will have to go through the trouble of tapping all four holes because if you only do one, you will be hard pressed to find a spring for the larger screw that has the same tension as the other 3 normal ones...
Good luck! Otherwise, I dunno if you can purchase a replacement back plate from thermalright themselves. Send them an email first!