Water column fertilizer will get into the substrate albeit not in any great amount. By the same token, the root tabs will diffuse into the water column, but again, most should be uptaken by plant roots.
Honestly, EI is the easiest way to dose, and adding root caps makes sure you are covering all the bases. I don't understand why you would need RO water to dose EI. I would just do EI + root ferts and a weekly water change. You can tweak a little here and there if you want, but EI takes away all the guess work.
Some don't like or want to use the EI method, and more power to them, but the dosing is easiest with this method, and the weekly water changes keep everything nice and stable.