LordHavoc has pointed out that under OpenGL nearly every driver supports GL_ARB_texture_env_combine or GL_EXT_texture_env_combine (older) for more varied multitexture blending effects than the ones I listed in my previous posting. This is good news for when I implement the OpenGL version of the renderer since it means I can support the same custom multitexturing modes as Direct3D.
Note that there will still be 2 lists of blending modes, those which are only supported in multitexturing mode (and thus require the application to specify a fallback multipass operation in the case of a lack of texturing units) and those which are supported in both multipass and multitexturing, which Ogre will derive an automatic multipass fallback.