Ok... so been visiting the bowl this morning and kinda missed a point about your post Weas. I responded to a different issue. I felt it was a good response, so I kept it and included it below just for educational purposes. If you were using 5gal buckets, I would have said those words.
Anyway... I kinda like your idea.... seems a lot of work for a solo cup hempy... but still. Maybe start by creating a simple gravity fed device... then covert it over to a pump on a timer.
I dunno how your medium will retain water. You'll have to eyeball that situation and see how fast a solo cup of coco dries out. You still want it to dry between waterings after all.
I did a little clone cup recently and treated it a bit like hempy. No hole in the side. Instead, the holes were in the bottom and I set the cup in a plastic lid that held about 1 inch of water. Kind of an external res. Normally, I'd worry about the light on the res, but since I was feeding (changing nutes) twice a day, there were no issues.
Do you have any pea gravel hanging around? Pea gravel might work similarly in a hempy cup that small. Be sure to bleach it first.
(start educational post....)
I think you're drawing outside the hempy lines pretty hard there.
Let's chat about watering techniques for a moment....
Our goals in watering a hempy are 3 fold.
First, we want to flush and fill the bottom res area with every watering. We do this by watering until we get a good runoff flow. When I see that Austin Powers movie with the peeing cherubs scene, it totally makes me think of my hempy runoff. I water until I get enough flow out the hole to cause the stream to project out from the container and form a good arcing stream. I apply the water evenly over the upper surface so that it drains into the res evenly and pushes out evenly... at least that's what I'm shooting for.
The second goal is simply to water the medium evenly so that all the roots get watered.
Last, but way not least, is the drying period between waterings. This is gonna be different for you, simply because of the difference in mediums. I'm not quite sure how coco is gonna work.... never done it before. I try to gauge this by weight and by finger.
So, having said all those words.... I kinda feel that an auto watering system has some real challenges. I feel you'd need at least 2 watering rings in every pot to get even coverage. Additionally, if one is watering every 3 days, ones nute solution would get old quick. One more... water needed changes at every watering due to humidity, temps, and transpiration.
I'd stay with hand watering if it were me. KISS baby... KISS.