> Could I easily add some extension code to get a message in this case,
> too? If yes, where is/might be the "start point"?

Probably

 
https://github.com/kschiess/parslet/blob/master/lib/parslet/atoms/can_flatten.rb#L87

Such a feature would be quite the opposite of what result flattening is 
about - and would probably not make it into parslet, unless done 
cleverly. As is, for example, the case with error reporters that can be 
swapped out.

cheers
kaspar