If you don’t have enough fuel to divert, nothing changes, except perhaps whether you call it a Mayday and put the pax in the brace position. I would only be going to that extreme if the runway was so short that an over run was likely. In the case of Wellington and a B767 I rather doubt that.
But if an over run WAS clearly inevitable AND a longer alternate could be safely reached by ignoring the FL200 limitation, I would rather go to the alternate.
The FL200 limitation does not mean that the aircraft will crash if it is exceeded.