Actually we already have categories for each version, which allows cross-referencing other versions if needed, e.g.: Category:OpenFOAM_Version_3.0.x - each category page does act as a wiki page.
What I mean is that we already have a very hard-time maintaining the existing content. Creating dedicated namespaces for each version would only make sense up to a point, for example, for keeping documentation on how to go from version A to version B. And still, that should be stored inside something like what we have for OpenFOAM_guide.
That said, if you already have a set of wiki pages that can be used as an example of the structure you have in mind, it will likely make it easier to see what you mean.