In June 2011 we went to Beza Mahafaly and I can highly recommend the spiny forest over there. Beza Mahafaly, including the zebu cart ride to it, was one of the highlights of Madagascar for us! On the way from Betioky to Beza Mahafaly you will already see a lot of spiny forest. In Beza Mahafaly we had a good guide that provided us with a lot of info on the medicinal uses of the spiny forest as well. I'm currently working on a travelblog of my previous journeys including Madagascar, on which I'll go into the details of the trip to Beza Mahafaly as well. I'll post a link when I'm finished, but here is some info already that might help you.
We went by taxi brousse from Toliara to Betioky (took 6 hours going up there, 8hrs back because of a flat tire). In Betioky we rented a zebu cart to Beza Mahafaly (took 8,5 hours going up there, but a lot less on the way back due to a different road and trouble with the zebu). So I'm not sure how long it would take by 4x4. We thoroughly enjoyed the zebu cart trip though! Madagascar National Parks says by 4x4 it takes 7,5 hrs from Toliara to Beza Mahafaly, but I think it should be less, because they say by taxi brousse it is 6hrs (correct if the bus doesn't get a flat tire) from Toliara to Betioky and 1,5 hrs from Betioky to Beza Mahafaly by motorcycle. I think the stretch from Toliara should be faster when going by 4x4 than by taxi brousse (a 4x4 Mercedes-bus).
When staying at Beza Mahafaly we had our own hammocks and we were the only visitors (besides the researchers), so I didn't see if there were tents for rent or not. The Bradt guide says it is possible to rent a tent, though. I think that should be arranged in Beza Mahafaly, as I can't imagine anywhere in Betioky it would be possible (most people just rush by Betioky in 4x4's). A guide can be arranged at Beza Mahafaly.
We spent 3 nights at Beza Mahafaly. We enjoyed both parts of the park: the spiny forest is really really special and in the other part you're going to see lots and lots and lots of Verreaux Sifaka's and ringtailed lemurs. When visiting the spiny forest you have to start early because it gets pretty hot up there, so I would advice visiting that part the day after you arrive at Beza Mahafaly.
In the spiny forest make sure you wear long sleeves, because there are no clear cut trails, so thorns will get stuck in your clothes. If you want to take a night walk (not in the spiny forest, but in the other part of Beza Mahafaly which is great as well both day and night), then the guide has to get permission. In that case make sure to bring your own light, because the guide doesn't have any. On one point the Bradt guide is incorrect, because there is filtered water at the campsite at Beza Mahafaly, so no need to bring your own filter.
We enjoyed a lot of places in Madagascar, but Beza-Mahafaly really is one of a kind and really stood out, so shouldn't be missed! I actually spent quite some time trying to figure out a way to fit it into our schedule including the zebu cart ride and in the end we decided to go there first, so when arriving in Antananarivo by Air Mad we took a connecting flight to Toliara (with 50% discount). Back from Beza Mahafaly in Toliara we took a flight to Mahajanga to continue the rest of our trip in Madagascar from there.
