It is standard practice to use the letter n after the page number, for index terms which are located in footnotes or endnotes. Examples: 29n = Entry term is found in the footnote on page 29. 127n3 = Entry term is found on page 127, footnote or endnote 3. (Details in the "Chicago Manual of Style", chapter on Indexes.)
Sounds reasonable but we have to consider cross-application compatibility. Indexes in footnotes are treated like in the text at MS Word as it's done in LibreOffice Writer (and we can open the documents mutually without loosing the format). So it's probably a WF unless the ODF definition explicitely allows it.