I've avoided SDL2 2.0.4 because (at least the last time I checked) it's only available as x86_64 on OS X, and Slayer Shock is built for 32-bit on all platforms.
That being said, it looks like the headers for SDL 2.0.3 do support the X1/X2 mouse buttons, and I've made the appropriate changes for that; but I don't actually own a 5-button mouse I can test with, so that'll be an experimental change whenever it comes out.