An excellent pdf showing some of the adapted bike hire opportunities in Belgium www.accessinfo.be/fileadmin/bestanden/docs/Cycling.pdf
In France wheelchair bikes are known as Velo Adapte or Rollfiets.
Near the Swiss border Colibrius rent a range of adapted cycles at Brognard, Pays-de-Montbeliard www.colibrius.fr
La Rochelle, North of Bordeaux www.maisondesvelocipedes.fr/?page_id=15
De Hoge Veluwe National Park has wheelchair bikes available free of charge www.hogeveluwe.nl
Star Bikes Rental Amsterdam has a wheelchair bike available for 18 euros a day
Near to Zaanse Schans and De Rijp you can rent a wheelchair bike for 20 euros a day in Wormer at fietsverhuurwormer.nl
In Barcelona they have the funky Batec which are worth checking out. An electric bike that clips onto your wheelchair.
Go to this blog post Where to Rent a Wheelchair Bike in the UK for a list of rentals, traffic free paths and links for throughout the UK.
Please add more Europe Wheelchair Bike info in the comments