You may get help here but it is pretty quiet right now. Here is a link Gardening in New Zealand.

All I know is that it is 10 - 15' tall and wide so might not fit in your space. It can be kept pruned to shrub height apparently. Good luck!

How much root room is there?

You aren't telling us how narrow the spot is. But I don't think you'd be happy with the appearance of a Feijoa that must be kept sheared to a narrow silhouette.

You might look at Podocarpus macrophyllus var. maki; it takes very well to shearing.

That is a very narrow space for most plants. Since you want to go vertical I would suggest an annual vine so that your planter does not burst at the seams, so to speak. Any chance of making the planter larger - much larger?