Determining solubility of a gas in a molten salt
If you know the phase diagram of the gas, and can calculate the exact pressure the gas should have given the volume outside of the molten salt, you can use the measured pressure/temperature to determine any difference in pressure. Then you can calculate the number of moles of gas that must be absorbed to generate the pressure difference. 
If the gas is relatively inert (which it probably needs to be if it wont react with molten salt), you can probably use the ideal gas equation:
$$P_0V = NRT$$
If you compare to the actual pressure, you can determine the number of moles absorbed by the salt:
$$\delta N = (P_0-P_\mathrm{exp})V/(RT)$$