Easy. Go to registry: HKLM>System>CurrentControlSet>Control>Video>(random string)>0000>DALNonStandardModesBCD1
Add resolutions 2x and 4x your native. (ie, 3840x2160, 7680x4320), restart your computer, set FSO to run at those resolutions.
Voila! Instant full screen super sampling anti aliasing.
(That was a joke btw. More than likely your gfx driver will fail if you crank it up to a resolution like that. If it doesn't, you won't have playable frames. Also, in general, adding nonstandardmodes that are higher than the default highest resolution isn't a great idea unless you know what the card's capable of)