HeroQuest Glorantha (2015, a.k.a. “HeroQuest 2.1”, not HeroQuest 1st edition, 2003) has simple contests for simple one-on-one conflicts, group contests for conflicts where a group cooperates against a challenge or another group, and extended variations of each. But none of the four contest types seem useful for contests like races or other competitions where everyone is competing against everyone else. Doing multiple contests of any kind in order doesn't seem right, since the action is simultaneous in these situations, and not between pairs anyway. I know about the guidance for Free For Alls (p. 111), but racers can't be paired off and resolved iteratively as in a grand mêlée so that doesn't help.
How are such multi-sided contests run in HeroQuest 2.x?
Why do I ask?
In specific, I will be demonstrating the game system soon, and there was a rock-throwing competition scene in the recent campaign that I wanted to re-run using HQ to show how a familiar situation works mechanically. But I couldn't figure out how to make it work as a contest. Obviously that means it's not a great situation to use for the system demo, so I won't be doing that, but it did suddenly make me concerned about running such things when they come up. There are enough such conflicts likely to come up in this campaign that I want to be prepared to handle them.
As usual, house rules are only useful when backed with experience
I'm specifically looking for how to model such a conflict using the existing rules, rather than speculative suggestions for house rules. (I have no trouble with coming up with experimental house rules I could try.) Previous experience attempting and failing to do this, and then detailing a house rule that worked to fix that problem, would be a good consolation answer though.