Heterogeneous Equilibrium Without Gas
You're right, they don't really have concentrations or pressure to work with, which would lead to 0/0 craziness. 

I think we can go back a step and look at how the equilibrium constant is defined. When you derive the equilibrium constant you don't use pressure or concentrations. You actually use the substances' activities which we can relate to change in Standard Gibbs Free Energy for the reaction (which is easy to find with tabulated data):



For a full derivation refer to Wikipedia's article on the equilibrium constant.