The OpenGL 3D plugin is already there, otherwise you wouldn't be able to see the OpenGL tab in the emulator settings or change any of the OGL-related settings there. The OpenGL "plugin" is integrated into iDeaS.
It could be that the save state code is buggy and doesn't work well with certain games that use lots of 3D graphics. Besides, you shouldn't even rely solely on savestates. Use in-game saves instead.
P.S. I just did a savestate test with FF3 and the error message pops up, it does load the 2D portion of the game (Status menu) but fails at the 3D graphics. This further proves my "theory".