If you watch VH1 Classic Albums: Hysteria, in the bonus footage of that presentation, Rick Allen and Joe Elliot actually go into this. Prior to their Donnington Festival appearance (their first major show since Rick's accident), they actually did a set of 4 or 5 shows in Ireland, where they brought in the drummer from Status Quo to help Rick out, in case he couldn't handle it. The first couple of shows went great, but on the 3rd or 4th show, that drummer couldn't make it to the gig in time, due to weather leaving out of London. Flight was delayed, landed on the east coast of Ireland, and took a taxi all the way to the gig (on the west side of the island), and got to the show an hour and a half late. Rick did all but two songs by himself, and didn't miss a beat, as the Status Quo drummer quietly snuck onto his kit. The crowds didn't notice a difference.
After that show, that drummer thanked the band, wished Rick luck, and went back to playing with Status Quo, because Rick didn't need his help anymore. The next show was Donnington, and the rest was history.
BL.