Justin,

You asked the following:

>Where is phase diagram information obtained?

If you search under Google you can find phase diagrams for water and CO2 from numerous sources. Unfortunately this isn't what you want. What you want are pressure versus temperature curve fits for water and carbon dioxide phase transition (the mathematical basis for the diagrams). This information can be found in thermodynamic reference books at a university research library (engineering library). These curve fits are calculated through statistical mechanics using a Gibbs (or Helmhotz) free energy solver. I have one of these routines in my atmospheric entry simulation code. This routine is based upon the Gordon and McBride code. The Gordon and McBride FORTRAN source code can be downloaded from the NASA Glenn Center. Get on NASA Glenn's search engine and lookup Gordon and McBride then follow the links. The Gordon and McBride code is among the best but I do NOT recommend your getting this code (it would be like killing an ant with a hand grenade).

>How is it utilized by computer modeling programs? (aproximate peicewise equations?!)

Piece wise polynomial curve fits is the standard method for thermodynamic modeling.

>Bonus: Where might I get emisivity data?

You're screwed. Planetary emissivity is strongly based upon cloud cover. Cloud cover is determined through weather simulation. As you know, weather simulation is a hard problem.

>the local library hasn't been much help.

Don't bother with your neighborhood library. You need to visit a university research library.

The question you didn't ask:

How do I calculate atmospheric heat storage through water vapor, carbon dioxide and methane green house effects?

Very hard to do. Many Ph.D.s have been devoted to solving this problem. Simply calculating the surface temperature of a planet through radiative equilibrium using the Stefan-Boltzman law will only work on planets without atmospheres, i.e. Moon, Mercury, asteroids, etc. Do the naive calculation for the Earth and you'll find the surface temperature is way to low. Why? Because of the water vapor green house effect due to the ocean. Water vapor is a neat temperature regulator. If the atmosphere is clear then heat is absorbed through water vapor. However if too much heat is absorbed then clouds form from the ocean thus blocking out the sunlight (self regulating). Also the ocean and ice caps act as nice thermal reservoirs. Isn't nature marvellous?

Gary