I just had an odd thought. Would it be possible to partially refill your co2 bottle by putting an active fermentation in a keg? I'm thinking no. The amount of pressure you'd need to liquefy the co2 probably makes it impossible. Even if it doesn't, the check valves would.
It's still an interesting thought. Maybe carbonate one batch with the co2 produced by another... I'm kinda tired tonight, and my mind is wandering.
Here's a fairly long thread discussing capture of co2. https://www.homebrewtalk.com/f11/how-about-co2-capture-system-48208/index3.html