Let me answer my own question here:
one needs to pass the variable as RooArgSet that contains the variable itself and the category to RooMcStudy. So it should look like this:
fitvar = RooRealVar( ... )
category = RooCategory( ... )
combined_model = RooSimultaneous( ... )
mcstudy = RooMCStudy( combined_model, RooArgSet( fitvar, category ), ... )