Hello OP,
I agree with the people who posted that the surface is your redundant air supply in your scenario. However, taking your pony for training purposes or to create "muscle memory" is a different story.
Remember, 10', 20', and 30' are deco depths for deeper dives. And usually, regulator tank systems give you warning before they completely fail--they usually free-flow. So, breath off the free flow and ascend slowly.
OOA at 30'? If this is the issue, please learn to control your breathing and to regularly adjust your dive plan to fit your air consumption.
Caveat: A pony bottle is a great idea while diving in your swimming pool, if you choose to dive with the pool covered! Any overhead issues should require the prudent diver to have redundancy.
My .02 cents, and they aren't worth spit!
markm