Looks like it's possible to force anti-aliasing for Nvidia graphics cards in the PC version of Episode 2 afterall. I don't have an Nvidia graphics card myself, so I can't test this, but someone in the Guru3D forums mentioned that this works for Sonic 4 Episode 2.
First thing, the tool Nvidia Inspector is needed:
http://download.orbmu2k.de/download.php?id=51
When you open Nvidia inspector, next to the driver version tab, there's a little icon with the screwdriver and a wrench, click on it.
After that, follow the steps in the picture:
1: Select the game executable, it should be located in steam\steamapps\common\sonic the hedgehog 4 - ep 2, and then it's the file Sonic.exe.
2: Write 0x000012C1 in the Antialiasing compatibility option.
3: Set the Behaviour Flags to None.
4: Change the Antialiasing mode to Override.
5: Pick the Antialiasing setting you want (I don't have an nvidia graphics card, so no idea what options it has).
6: Click in Apply Changes.