Sometime around 2008-ish we bought a DVD from a couple from the US (Ray and Carol).
http://www.caminovideo.com/ I just checked the website and it still active. Tracey just found it on Amazon:
http://www.amazon.com/dp/B004OR1KY0/?tag=casaivar02-20
They had a cart which he made which, from memory, looked similar to a golf buggy and they started from Roncesvalles.
Some way through their Camino their axle broke and needed welding. As highlighted by
grayland in the post above they also had problems negotiating some of the rockier downhill sections, but they seemed to appreciate not having to carry their packs. If I remember correctly, their purpose for using the cart was to re-live one imagined traditional approach to travelling along the Camino.
Anyway I recommend that anyone who is interested contact them through their website and get a hold of their DVD or via Amazon. It was good watching.
If your reason though is to save your back, then I second
wayfarer's suggestion to consider the pack transfer service.
If you are determined to use a buggy of some sort then I suggest that you get it as soon as possible and train with it over difficult terrain, up and down hills, over drains and cobble stones etc. You might find that the pulling and pushing of a buggy puts you at risk of further back damage.
If I were in your shoes and could not delay the trip I would go for the light daypack and pack transfer option. There is no rule that says you must move your own pack the whole way by yourself, especially with an injury.
If you haven't already perhaps its a good idea to consult your doctor and get some advice.
All the best with your Camino. I hope it goes well for you.
Jason.