As far as I'm aware, there are no freely available implementations of the distance-driven projector/backprojector (it is patented). You can, however, code it yourself without too much difficulty.
Start by reading the papers and understanding what the projector is doing. There are only a few key parts that you need:
- Projecting pixel boundaries onto an axis.
- Projecting detector boundaries onto an axis.
- The overlap kernel.
The first two are simple geometry. The overlap kernel is described in good detail (and mostly usable pseudocode) in the papers.
Note that you won't wind up with an actual matrix that does the projection. The system would be too large for all but the tiniest examples. Instead, you should write a function that implements the linear operator corresponding to distance-driven projection.