Why does SDLK_RSHIFT and SDLK_LSHIFT not give different results? For me, anyways, they both register no matter which shift I'm pressing.

You don't normally get different keycodes for the left and right modifier keys on the Mac. I believe there is a way of getting this information, but SDL doesn't use it. Also, some Mac keyboards are unable to return separate keycodes in any case, for hardware reasons.

Ok. Just for verification sakes, SDLK_LMETA / SDLK_RMETA are the CMD keys, right?

I'm pretty sure they are. Try them.