First, are the pots real stone, reconstituted stone or concrete? Pots with cement in them tend to leach lime into the soil and make it unsuitable for some things, but lining them with black plastic (with drainage holes) should sort this.
A self fertile skimmia wold be happy in this situation and as it only grows slowly should be OK in a big pot for quite a while.
It holds its red flower buds all winter and they open in spring, and then you get red berries in late summer through until the new flowers. They will be happiest in ericaceous compost.
Sorry can't think of any summer flowering evergreens for that position though. You could try sarcococca, again winter flowering, but there would be room to surround it with fibrous rooted begonias in summer.