I think a free standing stand is the answer.
I had put some thought into this and given up, but now you have me going again.
This is something I would really like to figure out becuase I do a lot of camping out of my truck in Arizona and Utah where there ain't no trees and I have come to very much like hanging in the hammock. Right now I use my cot, which is an excellent system as I can stop and set up camp leaving everything in the truck.
My pickup is a Tacoma, with no pockets for stake-poles. Doing something on the diagonal with a ridge pole might work, but you would have to carry a long ridgepole or figure out how to build one that would break down into sections.
I am thinking that the thread about the turtle dog stand is the answer the more I think about it. In other words, carry a stand in the truck that can be quickly set up where and when needed. I know this isn't actually hanging in the truck like was originally asked. It might be possible to use the truck in some way in lieu of one end of the stand, but a fully independent stand has a lot going for it, and could be useful at home too.
And you know, as I think about a ridgepole in a stand like this -- it only needs to resist force along it's length, so it could be two pieces of PVC that fit together. The center of the pole carries no lateral load.