I think you're planning on adding disaggregation of P to the tool, right? I can point you to my code that does this, if you haven't got it already.
In addition, it's possible to estimate fdir (fraction of solar that is direct) from tskc. This hasn't been done previously in VIC, but there's a fairly simple equation relating fdir tskc (which only you have access to, in your tool, at this point). I can point you to a reference for it.
If we incorporate that equation into the tool, then all necessary carbon inputs can be estimated without being supplied. Thus, your tool could produce all of these variables (Catm, coszen, fdir, and par) optionally if the user wants to run carbon in VIC.
Of course, these inputs are specific to the carbon stuff that I added to VIC. I don't know what your plans are for the CN module - whether that will become the official carbon module for VIC. Presumably it would need similar inputs, but maybe it computes them itself.
... and it turns out that I'm wrong; fdir is estimated in MTCLIM.
From @tbohn via personal communication: