Once in a while, you may want to (1) adjust or create a kerning pair (or, better yet, kerning class) in a font, or (2) adjust a particular glyph's sidebearings.
examples of scenario (1) would include a font that's lacking a certain kerning pair/class, such as when more ›exotic‹ variants of a glyph were overlooked by the designer:
W + a is kerned, but
W + á isn't. Or: adjusting punctuation spacing for a certain linguistic context, such as the famously loose puncutation spacing common in French. Scenario (2) would apply, for example, when a particular glyph is too close to (or two far away from) all its possible neighbors, whatever these may be. Such as the slash in an otherwise well-made font I recently licensed.
Obviously, you may edit the font file itself, but this may not be desirable for several reasons, and it may not be permitted by the font's EULA either. I'd like to arrive at a little survey that addresses questions like:
* what »non-invasive« methods are available for these purposes?
* what engines are they compatible with?
* what are pros and cons?
The two options that I can think of I've already posted as answers. Feel free to edit, as there might be more to be said about them.