Yes, and some owners have converted these to use derailleurs. One slight objection is that the trailing jockey sprocket shortens the path from the chainwheel are to the rear cassette, so lateral drag could affect the rear mechanism on an eight or nine sprocket cassette.
Some owners have used the Xtracycle rear extension with derailleurs successfully on the Lafree bikes, which substantially lengthens the chain run.
Jockey sprocket, here shown with bike upside down: