Is it possible? Can I extract the velocity data from an OpenEXR image and use that to generate motion blur? If so, what tools are necessary and are there any tricks or suggestions anyone has for creating the EXRs in 3ds max?
You need RevisionFX' ReelSmart MotioBlur. It has a "Vector" variation that can use velocity channels in the manner you describe. To extract the channels from the EXR files, you will need FNord's alternative OpenEXR plugins (which are free). I don't know much about MAX, but I believe the velocity pass is MentalRay specific, so all knobs and levers will have to be pushed there.
In Cinema 4D, you can render out separate files for each of your multipass layers. Having all your layers stored in one EXR frame is convenient, but I don't think it's the only way to go. Can MAX render out separate multipass files at once?