Like you , I have a few eyepieces with short eye relief and hard eye cups, what I've done is sort through my eyepiece collection and swapped soft eye cups from long eye relief eyepieces. Basically all it means is that the LER eyepieces have the harder cups now and the shorter E.R. eyepieces have the softer cups, obviously to be able to do this means that I've got too many eyepieces to begin with but I can live with it.
Chris, unfortunately due to the nature of different rubbers (natural rubber, neoperene, etc.), they can't be softened.
I looked into this about 2 years back.
Old hardened rubber can be restored to a slight bit of softness by using automotive compounds like leak-sealers, gasket re-juvenators, etc, but all of these will give off an oily substance for years, and fumes.
The other thing that can soften old rubber is Transmission/Power Steering fluid. Again the same problems apply.
I did read where some people used baby oil. They heated it up and soaked the rubber parts in it. But again it gave off an oily film constantly.
Don't bother with Armorall as it doesn't soften rubber. It just makes it look cleaner and can also give off an oily film.
The hard truth is, hard rubber products cannot be turned into soft rubber products.
But dry hard old rubber can be restored slightly back to its original softness, but with oily smelly automotive chemicals.
