Open pin in circuit, how do I represent that in a spice model subcircuit?

A good way to use spice is to add "parasitics" that have real physical values, like that of air as you have done. Another way this can be useful is to add small caps to pins that represent the PCB capacitance of a couple of pF's. (the .cshunt parameter places 1e-15 caps on every pin to ground to improve stablitity, this is done automatically unless it is disabled). Be careful because the solver has a "dynamic range" and using very high and very low values can affect numerical stability of the solver. Adding parasitics is kind of an art in knowing when and where to palce them, but based on physical "real world" values. Everything in the real world has some kind of capacitance, inductance and resistance.