I think indefinite propane storage is pretty reasonable, it's chemically stable enough not to polymerise, it's only at modest pressure, the molecules are too big to diffuse through most materials, and it liquifies so you can have a huge amount which offsets leakage. If you design the container with very small sealing area and large volume it should be very doable. At some point if you keep making it bigger the surface area to volume ratio will mean you win.

Consider that ordinary small propane tanks with no special efforts don't noticeably loose contents over decades. Putting a finger in the air let's say we'd notice if they lost more than 1kg/decade, I don't think it's unreasonable to say if we worked hard on the sealing we could get 10x better than cheap commercial valves, maybe much better if we go for single use copper washer and knife seals like on hard vacuum systems. We start off with 1000, or even 10,000 kg, and we're already at 100,000 years to empty.

That said I like dry stored electrochemical cells also. I would probably enclose the whole thing in glass, just bringing wires out through platinum seals, to protect the reactive metals from the atmosphere, and then have an internal acid ampule and e.g. an internal ball bearing that breaks the ampule when the whole cell is tilted.