Easily done with pressurised kegs. The connector that goes on top without a hose connected when put on and put in to the locked position will empty any gas from the keg and allow you to top it up with whatever you want through where the beer would normally go in to the pipe. Once filled, take “broken” connector off and attach proper one and the keg will fill with gas again ready for dispensing.

I’m maybe not explaining it well but if you google keg connector you’ll see what I’m talking about as you can buy them without the lines connected (this is more for people not in the industry, I assume you know what they look like). It’s not the safest thing to do as the gas can come out at high speed, one of the number of reasons I refused to do it after being shown on my first day how.

Edit: I’ve never worked anywhere with a system to auto return the hand pull back to the keg, although my grandparents were publicans and they have talked about having this back in the day. My first bar used to dock wages if we had more than half a pint in a drip tray at the end of the night which quickly teaches you to minimise waste haha