Juncus effusus 'Spiralis' (or 'Big Twister'). While these love moisture and will grow in standing water, they will also grow quite happily in regular garden soil, provided it is not allowed to dry out.

They make great container plants, so what you propose should work well. It is essentially just a container but sunk in the ground. The water holding crystals don't last forever, so keep that in mind.

It will also contain the plants nicely...........they have a tendency to spread.

Hmm, maybe then they need the soil more acid. They're in fairly heavy cascades clay but it doesn't stay super moist all the time