I have never tried this, but I don't see anything wrong with it offhand. I stopped doing the keg purge with StarSan because I found it was a waste of CO2 and of StarSan. I don't have any need for more than a gallon of StarSan at any time. Now I sanitize my kegs with a few cups of StarSan that I recover, then I purge them with fermentation gas. I am using the same CO2 cylinder I started in April, so I don't feel a need to be any stingier with it.