honestly unless you're putting a stainless airstone on the bottom of the dip tube, the CO2 still rushes to teh top of the keg (headspace). you'll get a minute amount of additional CO2 in solution that way...
IMO its not worth the effort since you're just going to shake it up, which is what really puts the CO2 into enough exposure to liquid for better absorption.
(and I stated this with years of experience running CO2 into freshwater planted aquariums...CO2 dissolving into solution is all about keeping the CO2 bubble constantly moving for several minutes...not the 3 seconds it takes to rise from the bottom of the keg to the headspace. if you had tiny CO2 bubbles from an airstone, these could easily dissolve before they reach the surface especially done as a slower trickle of gas)