I just looked through Mosher's Radical Brewing and found the equation for calculating the sugar content of liquors. I don't have the time right now to do the math for determining the Malibu/Appleton quantities to use but here is the example that is laid out for Frangelico:
Start by measuring the specific gravity of the liquor. You have to subtract the effect of the alcohol present, which is a known quantity (specific grav 0.789) and then determine the precise amount of sugar present. For every 10 proof add 1.06 Plato to the measured gravity of the alcohol.
Frangelico is 21.6 degrees Plato, 56 proof (28% alcohol/volume)
56 (proof) x 1.06 degrees Plato = 5.91 Plato --> correction for alcohol
Add for alcohol: 21.6 + 5.91 Plato = 27.5 Plato (% sugar)
Liquor quantity: 6oz by weight x 27.5% (sugar as Plato) = 1.65 oz corn sugar equiv.
So basically 6oz of Frangelico is equal to 1.65oz of corn sugar
I am going to update this in the next few weeks with the quantities of Malibu/Appleton used but feel free to PM me with questions.