Have you tried inputting the command in-game?
No, the EXE you use doesn't matter.I'm... pretty sure I am? I'm not using the Direct Draw version of the game.
srb2win.exe -opengl
gr_filtermode Nearest_Mipmap
sonic nameofsonicmodel.md2 3.0 0.0
tails tailsmodel.md2 3.0 0.0
POSS crawla.md2 3.0 0.0
srb2win.exe -opengl