LOL... I haven't even done my first grow in one yet, so I've got nothing but ideas, no personal experience. So far I've been 100% soil, my biggest move was from plastic pots to cloth! There are differences between this and hempy, I'm just not qualified to make the distinction.
You hit the nail on the head with tomatoes! Check out
this site... its all about self-watering tomato planters, and they have downloadable DIY instructions for how to make them.
(which should also work really well for mj, indoors or out)