After thinking about your question I would suggest one of two plants for your heat sink. Pyracantha, which should remain semi-evergreen. Or, anything in the Quince family. This is not evergreen in the winter but the bare branches have interesting texture.
There really is not much to discuss in the technique of espalier. You string several tiers of grape wire, secure the branches to it and prune the plant into the shape you wish.

fire thorn, pyrocantha work well

Have had some success with pyrocantha, espaliering it on my fence. you have to be really merciless and cut it back hard on the non-lateral branches. It is often done with fruit trees and is amazing how well they produce when properly trimmed. there are many books of the subject and i am hardly an expert. check out your local library, where they can be much more detailed that we can be here. good luck!

While not evergreen, Wisteria would provide fragrance and incredible spring beauty with beautiful racemes of either purple or white. It grows very fast (some would say *too* fast). My parents have this planted on an espalier on the side of their home in Russellville, Arkansas and it is absolutely breathtaking. People drive by, stop and take pictures of this beautiful display every spring. The bare vine espalier looks like a painting in the winter. It is truly a lovely sight alll year.

One word of caution about the wisteria...If you have a basement I wouldn't plant it next to the house. I worked in a green house for years and the roots are as evasive as the branches. But it is striking!! I also have one growing over a trellise.

I'm interested in an espalier of pyrocantha but DOWNWARD over a 8 foot retaining wall. There is nowhere at the base of the wall (only a concrete parking pad) to plant it but it would look terrific cascading down from the top. Can they be trained to do that?

The orange berry form would look great on the tan-brown stones of the wall.

Thanks for any help!

Yes there is a way... try working with more then one plant, same place or pot even, if you want to have it green longer but still have some show to it each year. I have had great success with doing that. Use any plant that is for your zone... or higher and of course likes heat. As for planting a plant to grow downward. Put it in a well drained pot of some kind...even fancy concrete that you make using the peet moss method as the sand. This will work for the plant you want on the concrete drive way too.

Here is a link that might be useful: break time (__)o have a LQQK see and a listen