Ok, so parts have been sourced:

I've mounted the big launchpad inside this handy case, since there is some travel in my future I should be able to take all of this with me and develop the protocol while I wait for the humidity/temp sensors to arrive. The water dispensing part of the build is going to need a hard look, as pumps are expensive, and solenoids are as well (at least at the indoor density levels I'm anticipating).

One possible solution would be to monitor humidity individually, but dispense water in groups. The software could detect clogs/failures by alerting if humidity didn't rise in all plants following a watering.