I have found some more interesting things about the viola caipira.
First, there is a homepage dedicated to the viola caipira with all important information about the instrument, its players, teachers, luthiers, some sheet music and much more: http://www.violacaipira.com.br/musicabrasileira/
Then I found another homepage with an interesting article about the history of the Viola Cabocla with some interesting pictures. This article was published in the Revista Sertaneja magazine during the 1950s.: http://www.widesoft.com.br/users/pcastro4/viola.htm
Another homepage dedicated to the Viola is: http://www.violaeponteios.com.br/site/
Finally the site of Eric Martins is also very useful if you want to learn to play the viola caipira – you can find chord charts and a basic method for the viola: http://www.ericmartins.mus.br/
The I have found that there are some big ensembles who play the viola caipira and sing. Those Orquestra de Viola Caipira sound very similar to a mandolin orchestra, they do also use the tremolo some times. Here is one example of such an orchestra:
Orquestra Paulistana de Viola Caipira
Finally I have found an interesting video that shows some steps of the making of a viola caipira in a small luthiers workshop: