Update: called Avis/Budget customer service at 800-352-7900 and chose the International option (the people at the President's Club number were of no use).
The guy there sounded surprised at what had happened to me and assured me I would get my money refunded. Faith is (temporarily) restored but we'll see how this pans out.

The lesson remains: watch out with bringing back cars late in Europe. If you can, extend the rental agreement before you pick up the car if you know you could be late.