Thorny things + people walking = bad idea.
But that's just me.

Just how wide is that area topside and how much pruning and training are you willing to do?
It is certainly possible to do it with enough time and some training.
But there are also tons of other beautifully cascading plants that you could use with the same effect and far fewer puncture wounds.

I think your Iceburg roses is a doable idea and would look quite nice. You have barely enough room , but just enough in regards to the width. ( in the photo below the planter is about 20 inches wide )
I live in Northern California and find that I need to routinely apply a systemic fungicide once every 4 weeks [April thru Sept] to battle black spot. I use Bayer 3 in 1 fertilizer/ pesticide/ fungicide ( comes in a granular form in a blue bottle ) This keeps the roses almost black spot free and blooming until Christmas day.
{{gwi:41430}}From California Gardening

I have never had any luck with training a bougainvillea to cascade downward. Their woody stems want to grow upward . In my experience I would say that you will have a challenge in maintaining a cascading / prostrate bougainvillea.

I do ADORE bougainvilleas though !! I find them relatively low maintenance in comparison to other vines, especially roses. They require little to no water once established and bloom their hearts out from May to the first few hard frosts .
They are thorny, but I only need to prune mine once a year - after the winter season to cull out any dead wood, and then sporadically during the growing season to keep a wayward branch in bounds.

Bougainvillea will seek full sun and overwhelm or at least aggressively intermix with anything in its way. Also 8b sounds a bit iffy for bougainvillea. You could pull it off if you are able to be very selective about the precise cultivar, but I couldn't tell you which to choose. Most are rather cold sensitive. Those that are tougher tend to be the more aggressive types.

Thank you for responding! :-)

Cearbhaill - Could you recommend other cascading plants you think would work?

Deviant-Diziner - I didn't know iceberg roses got so big. I thought they stayed small. Yours are beautiful, but quite a bit bushier than I expected.

I'm not really sure about my zone - my zip code is 91320.

New idea: If I do stick with the bougainvillea, perhaps I should plant it on the rode side of the wall and let it climb up? It wouldn't be interfering with anything that mattered on that side. What do you think about this?

My local Home Depot has some beautiful "Barbara Karst" bougainvillea for sale right now. I like the color. If you like this new idea, would you plant all the same color or mix it up?

Thanks for the help!

Jeri

Hoo boy, Barbara Karst. She's a big one. Per the Monrovia web site: "fast growing stems 20 to 30 ft. long". The color is great, and this eventually can become a tree-size plant. I recently removed a very old one, and the trunk was about 10" diameter. Be careful what you wish for.

Also rated for zone 10-11; look around your area -- do you see bougainvillea growing? If yes, that's a green light. If no, there is probably a reason...