I am a huge fan of the A Song of Ice and Fire series. I've spent hours getting lost in arguments about the politics, military tactics of ASOIAF, and I've spent just as many hours trying to predict what will happen in future books. There's a lot to discuss.
I think you've misunderstood the argument of the blog post that you cited in support of your claim about the wishing tree. The blog post's argument rests on comparisons between the tree and comparisons between descriptions of magic. The blog post correctly observes that when Martin describes magic, he keeps the details mysterious. Depending on the context, this creates a sense of wonder, horror, or both. The blog post isn't arguing that we will eventually find out the details behind magic in the universe, but that by keeping the details mysterious, Martin makes the magic more meaningful. Hence the reason why Martin seems to not at this by saying that knowing the origin of the tree would "spoil the mystery".
The blog post is not talking about predictions, which is what you seem to be talking about in this answer. As you know, one of the things that makes ASOIAF enjoyable is that there's all sorts of forshadowing and red herrings and you can make all sorts of predictions about future books. And of course people want Martin to spoil the fun, so he has to tell his readers to wait for future books.
The point here is that these two types of unknown aren't comparable. When Martin forshadows, he doesn't tell the reader to wait and see in the books; he drops all sorts of hints that allow the reader to guess what's happening. When Martin describes magic, he deliberately keeps things mysterious. We've learned all sorts of details about the politics and the plot as the books progress, but we haven't learned very many new things about magic.
It would be out of character for this trees purpose to be revealed later in the books. So no, I don't think the tree will be the subject of a great reveal plot wise. It's possible that I'll be wrong, but I doubt it.
In terms of making meta statements: if you mean "does Martin tell readers in his books (as opposed to in interviews, etc.) to wait for details to be revealed in future books, the answer is no. When Martin forshadows, he does so by giving the reader details that allow the reader to predict what happens in future books.