I've tried both pecans and walnuts, and the jury is still out on the success of both.
For both meads, I made an extract. I chopped up the nuts, but them in a jar and filled to just over the top of the chopped nuts with vodka, and let this sit for at least 2 months in the fridge (shaking occasionally). The vodka dissolves the oils over time, and is neutral in flavor. Grain alcohol might work better.
I'm hoping, as these age, the flavor of the pecans or walnuts will come forward. Right now the pecan is at at 1 year, and it's still very light. The walnut is at 5 months and I can barely taste it over the honey. (certainly, a milder honey might be better to use as well)
I have read that walnuts can be brewed into a wine without an extract as it has relatively low amounts of oil (I think Jack Keller has a recipe on his site somewhere...). Macadamia and Brazil nuts, on the other hand, are very high in fat and oil, so I think an extract is the way to go. Both Macadamia and Brazil nuts are very mild in flavor, too, so you may need a lot of extract, and a very mild honey for these flavors to come forward. But that's just a guess....I suppose we won't know until someone tries it, right?