You only have to "Enable access for assistive devices" for the in-game interface, because it requires directly controlling the hardware. In Windows this is done by dll injection, of which even a similar process isn't possible in any *NIX operating systems.
Linux's LD_PRELOAD system lets you replace some executable's dynamically-loaded symbols with your own implementations, and the OS X "dyld" manpage mentions "DYLD_INSERT_LIBRARIES" which seems to be similar. Is that close enough to "DLL injection"?
Fair enough, that wasn't a great example. However the Steam UI becoming randomly invisible when docked and then undocked, in-app browser constantly breaking, and using their own notifications UI that gets stuck on screen and doesn't offer Growl support are better examples.
