If a site wants to make multiple spec versions available, it should simply have one URL for each version. If your client needs an older spec version, make it use the URL that provides an older spec version. That could certainly be implemented by a ?spec_ver=x parameter, but it doesn't need to be made a core part of the specification.
OS version could be important in the future, but for now libpnd doesn't even check it. For that matter, I don't know how the OS version number given in a PXML corresponds to actual Pandora OS releases. So we can only really think about that here once it starts mattering to Pandoras in general.