It's not very wide, yet very long .... so i'd imagine you'd have to fill the lower half with something - a false base maybe and then plant-up the upper half.

12" wide for roots isn't a lot so it's going to restrict your choices somewhat, a lot of shrubs wouldn't be that happy in a small space but perhaps a small/short growing perennial.

If you are just thinking about temporary plants, some trailing annuals would be okay but then you've got no colour or interest after the summer.

I once planted an old milk churn with Vinca[ periwinkle] the varigated form and it was under the dappled shade of a Birchtree and looked nice with all the foliage spilling over and draping down the sides, nice blue flowers, at the end of each season i would cut back the older foliage sand new fresh foliage came through the following spring, have acheived similar results with lamium[ yellow arcangel], and with both types of eounymous, gold and green, julien.

I would be inclined to plant it with Ivy and stand it behind smaller pots of summer colour. Also it may just look very good standing empty?

The biggest drawback is the shady area. If there is nowhere else that it can go, cheat! We have one in a sunny area (up until 2pm) - we have several plastic pots that sit in the top and just constantly rotate them around as the flowering ebbs and flows. Dianthus, Californian poppy and trailing fuchsia are all used amongst others.

There are some good suggestions there for me to mull over.
Thanks, everyone :-)