There is commonly some room in the IGC system to permit some local
rules.

Local procedures, yes. Local rules, no.

Picture a pilot, whose first language is not English, and whose glide
computer assumes that the Turn Point radius for a Racing Task is 500
meters, as per Annex A. The pilot misses or misunderstands the 1000
meter radius declared at the Briefing. Now imagine that it is raining
on the Turn Point. The pilot pushes in to 500 meters and winds up
landing out. Everyone else makes the turn at 1000 meters and
completes the Task. The pilot complains to the Director, who rules
that the pilot should have paid more attention at the Briefing. The
pilot then protests the ruling, claiming that an illegal Task was
called.

In that situation, I would not like to be a member of the Jury.