I found this article (http://www.arrowriver.ca/dhamma/nimitta.html), which is interesting. It quotes from the Vimuttimagga:
To the yogin who attends to the incoming breath with mind that is cleansed of the nine lesser defilements the image arises with a pleasant feeling similar to that which is produced in the action of spinning cotton or silk cotton. Also, it is likened to the pleasant feeling produced by a breeze. Thus in breathing in and out, air touches the nose or the lip and causes the setting-up of air perception mindfulness. This does not depend on colour or form. This is called the image. If the yogin develops the image and increases it at the nose-tip, between the eyebrows, on the forehead or establishes it in several places, he feels as if his head were filled with air. Through increasing in this way his whole body is charged with bliss. This is called perfection.
I can confirm that
1) Breathing in a certain way causes this tactile phenomenon,
2) The tactile phenomenon is related to the sensation of breathing, but not strictly identical to it, because it can occur inside the body (perhaps this is Thanissaro Bhikkhu's "breath energy"),
3) "As if the head were filled with air" is a fairly good characterization of it,
4) Cultivating this phenomenon leads to pleasure, and this is how I have done it in the past (corresponding to my advice to perceive the "actual breath" and then to "breathe pleasure into" other areas; cf. point 2 of my previous advice)
5a) The tactile perception is soft rather than harsh (cf. point 2 of my previous advice)
5b) The stronger my concentration, the stronger, clearer, and softer the tactile perception becomes.
In addition, I perceive the tactile phenomenon as being composed of a particular visual image of small particles (which I ignore).
It is readily possible for me to go through everyday life with this perception (and this is what I had in mind when I described cultivating pleasure during the course of the day). I find that it arises very quickly for me whenever I intend to generate it, and its presence is associated with a higher-than-normal level of stability of mind.
Practically, if the generation of pleasure is still elusive, I suggest
A) Actively figuring out how to generate this perception,
B ) Actively figuring out how to strengthen this perception,
C) Actively figuring out how to spread it beyond the point where the physical breath contacts the skin
(The perception, once recognized, is fairly unique and does not aggravate the attention wave when pursued, whereas looking for outright pleasure may if not approached correctly.)
As a separate bit of advice concerning meditation in general, I suggest there is value in simply generating this perception and observing experience in a detached way. I have found it very easy to see (e.g.) the way that this perception is immediately replaced by forms of tension in the head when volitional experiences of any kind arise...it is a way to gain more sensitivity to what aggravates and doesn't aggravate the attention wave, and (subsequently) what habitually-fabricated phenomena can be let go of in meditation or outside of it.
Related to that, it is possible to use this perception as an object of meditation, and doing that will likely make clear the difference between "noticing" and "trying". "Trying" (directed at it or any other object) weakens it. "Noticing" does not.
If anyone is familiar with or becomes familiar with this nimitta phenomenon, it would be good for them to share what they know, and any tips on generating it.