I think keeping a very minimal furniture profile will help keep not distracting. You can do this by keeping large pieces away from the railing as much as possible. You can face the desk away from the railing so that the chair is all you can see if at all. A low back office chair can help too. Another trick to to use a glass desk and have storage against the wall. Lastly, try not too have wall art that can be seen from downstairs.

Here is a home office design we did for a client. It might help you to see it, so that you can move forward with your design process.

An Affordable Facelift · More Info

An Affordable Facelift · More Info

It might be of use for us to share a home office design of ours with you.

Home Office · More Info

You could try installing frosted tempered glass panels behind the railing. The light still comes through and allows for privacy or camouflage.