Clarify what if any steps are required to satify UFL terms for a font embedded in a (possibly DRM-protected) e-book

Another approach came to mind:
4) Include a sentence (or several of them) with a link as in approach 3), but inside the human-readable content of the e-book, such as on a copyright page (without the quotes):
    "A UFL[5]-licensed font XYZ is used in this e-book."
AND/OR
    "An OFL[6]-licensed font ABC is used in this e-book."

[5] http://font.ubuntu.com/ufl/ ([5] part would be replaced with a hyperlink surrounding the license abbreviation and pointing to the URL)
[6] http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=OFL_web ([6] part would be replaced with a hyperlink surrounding the license abbreviation and pointing to the URL)

P.S. Perhaps more specific sentences could also be used, such as:
"A UFL[5]-licensed font XYZ is used in this e-book for (such and such content).
An OFL[6]-licensed font ABC is used in this e-book for (characters from such and such language/script)."
Although that probably wouldn't be a requirement.
P.P.S. Approach 4 would be quite similar to what Google Fonts does on font pages, which also include a link to the UFL (or another applicable license).

Approach 5) could be as follows:
5) Like 4), but instead of linking to http://font.ubuntu.com/ufl/ or http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=OFL_web, it could link to an appendix page inside the e-book that contains the entire contents of such an external license page.

P.S. Approach 5) would end up adding 2-3 e-book pages per license, and could be considered a contamination of the e-books with unnecessary content. It would also entail English license pages in books in other languages, which could be confusing to many readers. And frankly, i don't think anybody has used such an approach for any e-book (or printed book).

Quick answer: use it as you would any other font, ideally with a short message in the imprint conveying useful information to the reader and telling them where to also get it from if they like it: eg.

  "Code samples typeset in Ubuntu Mono Italic 14pt. The Ubuntu Font Family is available from http://font.ubuntu.com/ and licensed under the Ubuntu Font Licence 1.0 http://font.ubuntu.com/ufl/ "

or to use a real-world example found in the found of pretty much every O'Reilly book:

  "The cover image is from The Royal Natural History. The cover fonts are URW Typewriter and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag’s Ubuntu Mono."

The intention behind libre/open licences is to give *additional rights* not normally available with most fonts: ie. allowing modification and sharing.

Thank you very much for an informative answer.

To elaborate a bit further, I am making the following conclusions (based on your answer, I'm pretty sure these are all true statements, but please correct me if I'm wrong):
A) Among the 5 approaches listed, approach 1) is sufficient.
B) Although it is not required, a nice to have is "a short message conveying useful information to the reader and" (possibly) "telling them where to also get" the relevant font "from if they like it".
C) The message mentioned in item B. can be placed anywhere in the book at the discretion of the author &/or publisher. E.g.,
C.1) In the frontmatter:
C.1.1) on the verso of the Title page[1], which could also be the Copyright page & have a "Copyright" heading (this page is also sometimes referred to as (frontmatter) Colophon;
C.1.2) in the preface;
C.1.3) in the introduction;
C.2) In the backmatter:
C.2.1) in the (backmatter) colophon (at the end of the book, which could have a "Colophon" heading).

[1] Which is actually where i put a short copyright statement at (& in my case it's page ii (page number not shown)).