I know how to calculate the density under normal ISA conditions at a given altitude, but I'm unsure about how to go about calculating it at ISA+10ºC, or at any ISA+XºC conditions.
Just going to answer my own question here, I found a very simple solution.
ISA + X ºC changes only the temperature. Pressure is ISA, because the model is static. So calculate ISA pressure with standard ISA formulae, calculate ISA temperature, add 10ºC, and obtain density through ideal gas equation. The ISA model presupposes that air is an ideal gas anyway.