Well, on Mac we have a maxwell's quicklook plugin. Which when installed let's you preview mxi files as other image files - gives a little snapshot of what's inside

But not the mxs files. I guess for that user should have an option to define what would be visible to a "viewer" or simply "tick" .. or Maxwell could automatically take a snapshot of existing cameras . .say put perspective camera in a priority and what is perspective camera pointing out is saved as a little jpeg and used as a thumbnail.
just thinking.
