I think the only time where a logical, explained magic system is really applicable for making solutions plausible aside from what you described, Neil, is when you describe the system of magic to such a degree that when some use of it is used as a solution that use is a logical consequence of the system even if that particular use has not been used or mentioned before, essentially making shelving unnecessary.
I just read over that sentence I just wrote, and my brain started to hurt. * blinks * Er... if you need me to rephrase that, tell me, and I'll make an attempt.
Actually, though, I disagree with both of you, to a degree.

I agree with you, Neil, in your rephrasing of the law, however I don't necessarily agree with you about your description of the way to apply it. I don't think rules
or shelving are the only ways to make sure the reader's sense of plausibility is high enough for the author's solving of narrative conflict.
To give an example, the one that started me on my thoughts about this, there is a Japanese movie called Seven Samurai that I've watched several times (with English sub titles, of course). One of the samurai is a man named Kambei, who serves as leader of the group. He knows a lot, and he can do a lot. At the end of the movie, he picks up a bow and takes down several enemies in the climactic battle, despite the fact that he had never been shown to be skilled in the use of a bow before, and that it had never been mentioned either. But I noticed that it didn't make me feel cheated to have him do that.
I think it was because the story had previously gotten across such an idea of the man, his almost mysterious talents, his wisdom, his huge experience, that, in a way, I was expecting him to surprise me, if that makes any sense. I was elated and excited when I discovered he could use a bow, not upset with the story, because to me it was perfectly plausible that he might have that skill.
I think that is one reason why the magic in LotR worked for me, even though it had no visible 'system'. When Gandalf did awesome things, I felt it was perfectly plausible, because I
expected him to do the unexpected and the unknown. Same with Galadriel.
If Tolkien had made Gandalf completely powerful and able to do everything and solve all the story problems, then I would have had a problem, of course, because that would have meant no conflict at all. But he wasn't. He failed a dozen times, other people could do some things better than he could, and there were more powerful people than he.
The point, though, is that I think it is possible to make a 'system of mystery', or utilize 'placeholder shelving', or whatever you want to call it, when creating the plausibility aspect of your story.