Hi Ed,
Yes, aquatic plants are terrestrial in origin and in fact, in the emersed state function as terrestrial plants but there are some adaptations that allow them to function efficiently submersed. It's difficult to compare behavior and efficiency in each state simply because of the different media. One example is that terrestrial plants have pores (stomata) distributed on the leaves by which they are able to absorb CO2 directly. These pores also allow evaporation at the leaf surface which facilitates, by capillary action movement of water and nutrients to be drawn up through the roots to the leaf. This is a powerful hydraulic force which enables tons of water to be lifted from deep in the soil to several stories the top of a massive oak tree. Capillary action in air works tremendously more efficiently then in water, where as you point out, the uptake mechanism is via osmotic pressure and concentration gradients.
Since a submersed aquatic plant has no pores, osmotic pressure and concentration gradients work to its advantage across the leaf structure directly as well as through the roots but is more effective through the leaf, if for no other reason the small distance to the destination chambers. The structure of the submersed leaf is really quite a bit different than that of it's terrestrial counterpart and that's one of the reasons that the submersed form of many aquatic plants are completely unrecognizable from their terrestrial form.
Through their basic rigid structure and through the opening and closing of the pores, terrestrial leaves are designed to modulate water transfer across the membrane depending on ambient conditions, so yes, applying nutrient mist on a terrestrial leaf is analogous to skin absorption, but a nutrient transfer across a submersed leaf form is a completely different story.
Apart from the scientific principles, the empirical evidence shows for example that it is possible to achieve nearly maximum growth rates with aquatic plants grown in inert sand and applying water column dosing only. I don't think that would be possible if aquatic leaves were as inefficient as the foliar feeding (or skin absorption as you say) in terrestrial plants.
I agree that theoretically, imbuing the soil with additional nutrients should boost plant growth, but isn't that what root tabs are supposed to do? I just don't think that they work as well as just adding more nutrients to the water column, so there is a practical element to my cheesy analogy.
I also totally agree that it's much better to have both a nutritious soil as well as water column dosing for maximum efficiency, however I think, based on the Barr experiments, that as the lighting is pushed upwards, water column dosing to supplement the basic substrate feeding would have a better yield that solely boosting the level of nutrients in the substrate.
Cheers,