Your questions are very broad, and have no definite answer. As mentioned in other answers, "hardness" doesn't have one single meaning.
The Wikipedia Hardness article mentions 3 main "types" of hardness, but even using one style of measurement machine, quite different (and conflicting) rankings will be observed. A hard material, for example, is pretty much useless if it softens in the rain or upon exposure to sunlight or just as it ages. Of course, you can protect a surface from sun, and rain (to some extent), but there are a fairly large number of properties that a substance must have to be "useful".
Also, and full disclosure here, I'm not a botanist, and have no knowledge of the Buloke, but Wikipedia says it's an Ironwood species. The same table that lists it at >5000 lists Ironwood at ~3000. You have to be very critical taking these numbers at face value. Ironwoods, I know a (very) little bit about. One of their properties is their high oil content. This is good for water (and bug) repellency but not good at all for painting or contact with other surfaces if they're prone to staining as (not if) the oil bleeds out.
As previous answer says, don't confuse hardness with strength. My guess for the "hardest" wood we could breed/engineer would be that its just as hard as the hardest biomaterial known. I think (but am not sure) this is either calcite, aragonite, or the stuff our teeth enamel is made of, hydroxylapatite. It would be interesting to determine if silica-based biomaterials were any harder, I wouldn't be surprised. (Diatoms and Radiolaria make silica walls). Since biomaterials are nanocomposites, and can be 10 times "harder" than the inorganic mineral they derive from, it's not really possible (imho) to say what the upper limit is for hardness.( Diatomaceous Earth is used as an abrasive, so it's probably pretty hard.)
For a material to be useful it not only needs a slew of properties to match a particular need, but the economics have to be favorable (meaning supply of the material good, and demand also strong).
The test you mentioned was (probably) designed (at least it was selected) to be useful with wood in the applications wood is used in. Meaning other measures would probably be required before a particular wood is deemed hard enough to function in some unusual, atypical way.
You ask two questions. The answer to the first is A. As far as what is now known, Wikipedia editors know more than I do, B. As far as what is possible, well, that's pretty much open-ended. Its certainly possible to make a plant develop a skin similar to the hard materials found in the animal (and microbiota) kingdoms. Find the hardest biomaterial known to man, and you can start there. If you want to speculate, increase its hardness by 10X.
To answer the second. You didn't give us all of its properties. As I said, giving us a single property and asking where it "might" be useful isn't likely to garner many incisive answers, its just far too broad and vague a question. As they say, the devil's in the details. Hard materials, generally are used to protect other materials from damage, or just the opposite, they are used to damage other materials. So, use as surface layers or in abrasives would be my first inclination.