Whalerider,
When doing the welcome PA I always stress out why we have have a delay. So when we have received slot because of French, Italian or Spanish ATC working slowly or actually being on strike I will tell them so. It may not be on the website for the public, but I can assure you that my pax having the delay will be told where the delay is caused. I am sure if not all- most pilots do so!?
If we have a substantial delay I will do the PA in the cabin giving them the reason for being delayed...that seem to calm people down.