This question is quite old, but I thought I would answer anyway.

In principle, this is simple. You add your slag composition, and then add a large amount of sulphur that is beyond what can dissolve in the slag. Thereafter, you can determine the sulphide capacity by calculating the sulphur concentration in the liquid slag.

You can also do this calculation with ChemAppPy, which makes it simple to group phases together to include liquid slag and a number of solid oxide phases if this is what you need.