I've seen promos for alleycats with "slow race" as one of the events. How is this conducted? I assume riders cannot trackstand, but how is this enforced?

well, since no one has replied to this day old thread, i'll just reply and say i have no idea whatsoever. maybe they make them ride on some kind of banking, like a hill sideways and see who can go the slowest? That's how racers jockey for the better position in match sprints at the velodrome.

i guess if the race gets that slow they might have to take their feet off their pedals

I don't know how they run this at alleycats, but in the old days when every bike was a fixed gear (like in the 1880s, I mean), bicycle races and exhibitions would always have a slow race, mostly for comic effect, in which riders vied to finish a course last without falling over. Of course, they were riding high-wheelers, so putting a foot down wasn't even an option - you really toppled in a big way and there was real injury potential. This was also an era in which people liked to bet on how many rats a dog could kill in a certain period of time.

They probably just have judges who boo and holler whenever they think someone is doing something wrong.

found this here https://www.qcw.org/kob/

"Slow Race – A box is created on the ground and riders enter the box on their bike and try to remain in the box for as long as possible without putting their foot down. Longest time in the box wins"