Hmmm, I think you might be overanlysing this, although I would love to hear what some of the others have to say about this too. My "interpretation" of the long tail fwiw:
The "long tail" should come naturally in your writing if your pages are topically focused and written for the average user (within your target market of course). So, in your example, you might only use that exact
phrase once or twice in your body text, but if that page's content is well rounded, other closely related phrases will naturally occur in the copy. And the tail will "lengthen." A good thing, right?
(I would also guess that the search engines likewise appreciate a "long tail" approach to content. i.e. why couldn't their algos analyse the semantic proximity between various topically related terms and then make a judgement on that page's relevance to a particular subject matter?)
So, I'm not sure if you would be "optimizing" for the long tail per se--at least not in a formalized manner--you are rather relying on the long tail to sort of inadvertently attract those surfers who are seeking something very specific (and thus are more likely to convert when landing on your site via these highly specific search terms).
This is at least "my" interpretation of writing for the long tail.
But I would still like to hear others' comments on what I think is a very fascinating topic!