Edit (5/24/08): I just came across a trailer design for pulling a wheel chair on a trailer. See Tony’s Trailers for more.
For handicapped passengers that are also capable of assisting in pedaling, Morgan Cycle offers the Caboose Trailer. This sturdy looking trailer bike can hold passengers up to 200lbs. It is similar in design to single-wheeled trailer bikes but offers the safety of a two-wheeled design.
Another trailer bike design is a custom recumbent trailer displayed on the designer’s website.