I agree that the increased solubility of CO2 will add to the suckback effect, but I have no idea how to calculate that.
In terms of when to attach a balloon, fermentation of 20 liters of 1.060 beer with 75% attenuation produces about 450 liters of CO2, so collecting 0.5 - 1 liter would mean just catching a tiny percentage at the very tail end of fermentation.