Great suggestion! It reminds me of one I saw somewhere, but I can't find that link.
It combined spot composting and compost tea with the kind of deep root watering you just described.
The person buried the bottom of a large PVC pipe 18-24" deep, with many perforations below 6" deep. He or she might have sealed the bottom of the pipe with a cap. The pipe was somewhere between 2" in diameter and 6". It stuck up above ground at least a foot or so, but I thought it would be cool to make it tall enough to use as a hand-hold in some part of the garden where footing was unsteady.
Then they filled it with compost makin's, or compost that had just barely started to break down. Then they put some cap on it so it wouldn't totally flood every time it rained.
Each time they watered, they filled the pipe with water. It would keep the compost damp, and create compost tea as it trickled down. Then the tea would escape the pipe between 6" deep and 24" deep, delivering water and compost tea to right near the roots.
And it probably attracted worms!
As the compost broke down, more could be added.
