I may be wrong, but was there not the rather bizarre situation of a Norwegian Registered Caravan operating Royal Mail night flights from Orkney and Shetland to the Scottish Mainland a couple of years ago? All was going well until it fell off the end of a runway somewhere. I seem to remember something about that.
There was also an enterprising chap who operated a Caravan for overnight newspaper transport. Apparently, he would buy a load of newspapers from a distributor at Luton for ₤1, operate the flight as a private flight to Glasgow or Edinburgh, then sell the papers back to the distributor for ₤1 at the other end. I believe this little idea was hounded out of existence by the CAA. Something about it being within the letter of the law, but not respecting the spirit of the law. (It may well have been Cherie Booth who was the QC representing the CAA).
It would be great to see the Caravan operating as a freight feeder service in Europe, but with JAA FCL rules requiring >700 hours before a pilot can operate in a Single Crew Environment it does make me wonder where the companies will find pilots to fly them. Similar question mark relates to the Farnborough F1.