Unfortunately, not without hacking the CR2. HOWEVER... As long as you don't use any nodes except 2D textures, and only use them on the Texture, Reflection, Bump, and Transparency bits, with the color swatches only on the Diffuse, Highlight, Reflection, and Ambient bits, they open in P4 just fine. The big problem here isn't so much the material nodes. It's the lack of an rsr thumbnail in the library. I'd also reccomend making sure you put the obj in a subfolder of the Geometries folder, and point the CR2 to it... P5 and PPP tend to put the geometry in the character folder, and that iritates many people, who never put the CR2 in the default folder anyway, having created their own filing systems.

Remember, kids! Napalm is Nature's Toothpaste!