A few people replying to this post have correalated a pony bottle system for use in an out-of-air scenario.
I disagree with that strategy for a pony bottle. Pony bottles should be used for emergencies when you suffer an equipment failure or when equipment is damaged by impacts or hose cuts during the dive.
People who need a pony system for out-of-air scenarios should be diving in less than 40 fsw until they can master proper breathing techniques and ingrain a systematic mental regimen for checking air consumption and nitrogen loading, IMHO.
Any recreational diver who uses more than 13 cubic feet in a slow and controlled ascent, which includes a safety stop, from 130 fsw needs to critique their air consumption habits.
I have done practice ascents from depth on 6 cubic feet with air to spare. That included an ascent rate of 30 feet per minute to 20 fsw with a one minute safety stop at 20 feet followed by a three minute stop at 15 feet. I am 6 feet tall and wiegh 190 lbs.
I like the idea of "clipping" the pony system to your rig with a boat snap which makes it very easy to pass the pony system to another diver.
markm