In past projects I tried wintergreen oil and several other liquids/combinations that can allegedly soften old rubber. I found that while wintergreen worked initially, a week (or less) later the rubber was a hard as it was prior to treatment. So my experience - admittedly with "commonly available" items - was that "restoring" rubber parts made them flexible enough to be re-attached/re-mounted but they returned rather quickly to their hardened state.
On the good side, if you don't plan to regularly remover/re-attach the particular rubber part, temporary softening might be all you need.
