That's not an easy one...most exotics will die down in winter or are just too big. You could try Delosperma as a trailer but apart from that, everything is either dormant or doesn't flower.

One of the smaller phormiums or cordylines might be good as a centre piece with the Delasperma round the edges.

I wonder if some small evergreen ferns would work for you?

A small phormium is evergreen and would fit in with your theme

The trouble with the hanging baskets that size is finding something small enough. I suggest ferns, dwarf ones. If they are going to stay outside, then you'll need hardy ones (obviously!), so I'd look at spleenworts, woodsia, and polypody