DC Current Density and Voltage in a Finite-Length Conductor
I would use a finite element analysis. Break the bar into small cubes of material and then model each cube as six resistors, all connected together at the center and one connected to each face. Assume that your two points are at the exact centers of two cubes. Use SPICE, or construct the resulting resistor network and calculate the current through each resistor.

This is a numerical approximation, of course, but it gets better as you increase the number of cubes (i.e. make them smaller).