While I was tempted to hang around to see if they eventually pulled anything out of the way, the heat of the day was beating down pretty hard by this point and I didn't want to risk sunburn, so I headed home. As did all the boats in the lock, who returned back into Cumberland Basin and presumably eventually back to the city, refunding/rebooking their passengers.
I found out that my decision to leave had been a good one two days later, on Tuesday evening, when I happened to be on my walking commute home during another locking out, this time with the lock gates working. I asked the lock keeper about the problem, and apparently they'd tried hooking out whatever was fouling the gate for quite some time on Sunday, but eventually gave up and sent divers in on Monday, who pulled out a large section of chain-link fence that was fouling the south lock gate and got everything working again.
GPS Coordinates:
51.44909, -2.62288
Location: Hotwells