Broke the mounting bracket for the CPU fan on my Asus M3A78-EM

Whether it will work or not will also depend upon the HSF you are using. If you are looking to replace the backplate only, then that should work. If you are looking to replace the entire mount, it would be prudent to search for your HSF's mounting system on E-bay or somewhere simular. I think it would work though (may require a little ingenuity) but I'd try anyway.