Looks like a fun project! Gonna have to say Paul is on the right track with the Morning Glory. I'd skip that one for the bed there and use it somewhere it where it can grow without check. One of my favorite places I see people use them is the small 1-2' mailbox or lightpole(private) beds, with a wire trellis to climb.
If you put them in that bed, their either going to climb the fence, or the other plants.. Maybe both. The beauty of keeping them in a small solo bed, especially surrounded by 'mown area', is they will come back year after year if they aren't hardy in your area.. Bonus in a solo bed, problematic in a mixed bed.
Are you in hard clay? Daikon radish might be a fun add that would help amend the soil.. Not sure if they all have purple flowers, but those I have from Outside Pride do. Small flowers, huge radishes. Not sure how the flavor is, as we mainly use them as a no-till soil amendment.. I would guess they are the Elephant Garlic of the Radish world, based on what I've read.