I'm seeing a range of possibilities here. E.g.,
Nothing special needs to be done, based on  (and UFL being close to OFL), , and an opinion stated under Use an Open Font.
Perhaps such font licenses can be mentioned in package descriptor (.opf) file,
using elements like
<link rel="cc:license" refines="#cover" href="http:// creativecommons.org/licenses/by-sa/3.0/" /> <link rel="cc:attributionURL" refines="#cover" href="http:// en.wikipedia.org/wiki/Simon_Fieldhouse" /> <!-- cover meta element included for 2.0 reading system compatibility: --> <meta name="cover" content="cover"/>
(that can be seen for a cover image in ) (but in this case, the license won't be seen by the user unless they extract the epub's contents (which would not be straight-forward if the epub is DRM-protected)).
On the other end of the range, perhaps a link to OFL and/or UFL license could be included on the sites on which the epub will be distributed. E.g., something like the following on the e-book's page on kdp.amazon.com: "A UFL-licensed font XYZ is used in this e-book."
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-licensed font XYZ is used in this e-book."
"An OFL-licensed font ABC is used in this e-book."
- 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 a page inside the e-book that contains the entire contents of such an external license page.
5.1. The linked page could be NOT included in any nav or ToC elements, but available within the e-book if clicked on.
5.2. The linked page could be NOT included in the book ToC, but included in EPUB2 guide (e.g., ), or perhaps EPUB3 equivalent landmarks element.
5.3. The linked page could be in the appendix, and included in book ToC.
Which of these or other approaches would be the easiest (minimalistic) and legally safe? E.g., is approach 1) OK as most opinions I have come across claim?
P.S. Perhaps more specific sentences could also be used, such as:
"A UFL-licensed font XYZ is used in this e-book for (such and such content).
An OFL-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).
P.P.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 think almost nobody has used such an approach for an e-book (or printed book).
 In URLs, replace "// " with "//" when applicable, as the site doesn't let me include more than 2 links. Also, note that my question is whether any steps are required at all, & which approach is the minimally legally sufficient one, which is different from a couple of related questions which ask how and where to include font licenses. This licensing-heavy question wasn't considered relevant on Stack Overflow, so I'm asking it here, in the hope of nailing the minimally legally sufficient approach for OFL/UFL-licensed fonts.
 http://font.ubuntu.com/ufl/ (if the site allows it,  part would be replaced with a hyperlink pointing to the same URL).
 http://font.ubuntu.com/ufl/ ( part would be replaced with a hyperlink surrounding the license abbreviation and pointing to the URL)
 http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=OFL_web ( part would be replaced with a hyperlink surrounding the license abbreviation and pointing to the URL)
 Note that guide is deprecated in EPUB3: http://opensource.stackexchange.com/questions/2844/how-to-properly-include-a-license-file-within-an-epub