Absolutely agree. It's possible in irops situations. I've only done it once when our YQR-YYC leg was cancelled on a YQR-YYC-LHR flight due to a bird strike on approach (an actual visible from the terminal chunk of the leading edge of the plane was missing!). We were rebooked to YYC on AC but we would have missed our connection. We walked over to Westjet and bought two one ways on their impending departure and then back to the AC counter and told them what we were doing. It was very easy.