An important aspect when using cfdemSolverPiso is that the particles must be smaller than the mesh cells in OpenFOAM.
In OpenFOAM one often does a pseudo-2D simulation where there is only one cell in Z-direction.
How correct would it be to make a pseudo-2D simulation in openFOAM and couple it to LIGGGHTS where the particle diameter are near the cell size in X and Y direction, and just increase the Z-dimension of the CFD-mesh? The LIGGGHTS simulation only has a "sheet" of particles which remain on their Z-position (very dilute, almost no collisions), so one can pretty much set the voidFraction by increasing the Z-dimension.
Cheers,
-A