Couple of questions if you don't mind? First, why the capillary matting? Do you have hundreds of seedlings so that just normal bottom watering as needed would be too difficult?

Capillary matting is notorious for over-watering plants and causing root rot. That is unless you let it dry out well between watering. It also tends to build up salts and bacteria on the matting over time so it needs to be well-disinfected on a regular basis.

Second, given that young seedlings need so little feeding/fertilizing and need it in a well-diluted strength, how difficult would it be to just lightly water them with a diluted liquid fertilizer once or maybe twice before transplanting them to the garden?

But if this is the way you want to go then I'd say, yes, take them off and fertilize off the matting. Trying to mix it in the water means the mat will bind it in the mat itself so minimal if any will get to the plants.

Dave

Thank you very much Digdirt, This year I just wanted to try it.