Hmm, I've never actually skidded in a fixed crit so maybe the powertap hub could work.
It's not just skidding though. I once gave a decent amount of resistance through the pedals of my track bike because I wanted to make sure I made the ramp at the velodrome, and the rear hub started to unwind itself. Not a nice feeling. So slowing down vigorously is enough, and I'm assuming in a fixed crit you may need to slam on the anchors occasionally.
It's not just skidding though. I once gave a decent amount of resistance through the pedals of my track bike because I wanted to make sure I made the ramp at the velodrome, and the rear hub started to unwind itself. Not a nice feeling. So slowing down vigorously is enough, and I'm assuming in a fixed crit you may need to slam on the anchors occasionally.