PLA foam is an excellent start. It could be better than polystyrene foam, because PLA contains more oxygen to form gaseous compound with carbon. I could only imagine POM-H (polyoxomethylene [-OCH2-]n) foam leaving less solid residues.
Within a ceramic mould deposited on the foam, I expect very little air mass is available, so the compounds present are together very reductive. I expect CO rather than CO2, which will pick some hydrogen to form things like H2CO. The rest of PLA [-C3H4O2-]n would be four H for one C if no hydrogen combined with CO.
If the foam has closed cells, the foaming gas contributes to the gross composition. With open cells, it will be air. The amount isn't huge, but it can increase radically the toxicity of the evolved gas. If polystyrene is foamed by a halogenated gas, this makes toxic fumes.
What do the up to four H for one C do? I won't risk a prediction. There will be some graphite and high-carbon compounds, but the amount doesn't result from a simple reaction equation. The rest can evolve as CH4, C2H4 and thousands more.
Out of curiosity, I've let Propep compute the equilibrium products of hot polymers brought to 1400°C and 1atm. This is NOT what happens in your case, because the casting process is very far from an equilibrium. Also, iron will catch some oxygen. The evolved gases recombine as they cool down, for instance CH3, H, HCO disappear for sure. These are molar fractions. Propep hides the least abundant species. I've take polybutadiene for its small H/C ratio. Even smaller at polystyrene, but Propep failed.
Propep failed to find an equilibrium.