So, I take it is only the "weeping" trees that have a tap root type system.

Resin, (or anyone will to chime in) if your were going to plant an "anchor specimen" 5 feet from a 3 foot high retaining wall (the top side of it, not the bottom), what would you recommend?

There is nothing about a weeping tree that makes it have a taproot. Often, oaks, hickories, and such do have taproots, especially if growing in sandy or gravelly soil. But.........most of their roots will still be near the surface.

As to what to plant, give us a little idea of what growth form, size, etc you are after. There's a lot of trees out there;^)

+oM

You may want to do some research into the crape myrtle. I've heard that they have roots similar to a palm's. I've heard it from a fairly reliable source, (it was in reference into fertilization), so you may want to explore that bit of info further.

Palm roots actually can have a considerable spread - 30-50' from the trunk on well established specimens - so I'm not sure what relevance that would have in this situation. I'd also be interested on what basis the comparison of crape myrtle roots to palm roots was made, as other than the the crapes producing a very dense network of significant roots beginning immediately adjacent to the trunk and just below the soil surface, as do palms, I don't see much similarity. Palms have an ability to regenerate almost their entire root system, so can be transplanted with a minimal rootball of 12-18" surrounding the trunk. I'm not sure I'd recommend something quite so harsh with a crape of any size.

Japanese Blueberry tend to grow down

I think you will find that most of the literature disputes this :-) Like all the trees discussed previously, the majority of the roots go out, not down. And they are quite inclined to lift concrete or push out a non-stable retaining wall.