nobody will be looking directly at the house, either walking or driving by they will see the hedge regardless if there is a wall in front. I wouldn't put the bricks all the way around the letter box unless a builder has structured it correctly. You could do the base the same as the res of the fence, set the letterbox in that base/footing and leave around the letter box blank to show some of the hedge.. Bad drawing attached..

I agree with Amanda and RWalton: Definitely black but I too question your use of a picket fence with a 60's cream brick - it will never read as 'right'. A paling fence, a metal slat fence, a combination of the two; but not pickets - they are from a totally different era.

Perhaps it varies depending on region (factors like climate and soil type) but our viburnums didn't reach privacy hedge size for years. And then when they did we had to start pruning them regularly especially beside our driveway. The space you have looks much narrower than ours so I think you would have a lot of maintenance to do if you planted viburnums.

What is the intention of doing something with it? Is it because you don't want to be hauling the mower up there every week in the warmer months for an area you otherwise don't utilise or are you fine with some mowing but want to make it a more useful area? What's your climate? Which way does it face? For example would there be any benefit from having more trees on that side for summer shade? Is there an easement? If there is you need to keep that in mind when planting over it.