Calculating resistance for piece of metal
Once you get beyond approximations from using \$R=\frac{\rho L}{A}\$, the math gets pretty hairy to do it in closed form. The current flow depends on the electric field, which depends on the shape (boundary conditions), to varying degrees (the resistance from 1mm probes through a sheet of 3mm aluminum doesn't change much as you change the shape of the aluminum past a couple tens of mm). 

I think the most reasonable approach for a complex structure would be to use a software package such as COMSOL and calculate it numerically. Or build a physical model from a material of known and consistent resistivity and test it (especially practical if it can be reduced to a 2D model). For simple situations it might be possible to mesh it manually and create a matrix of resistors, and then use SPICE to get a solution (when your only tool is a hammer..)