Use a regular character as you can't guarantee the font for your special characters will be used.
HOWEVER
In your font embedding attempt in odt/docx you need a character style that overrides the paragraph style. This becomes a span in the HTML. Or code manually
<p class="indented"><span="special-f">¬</span>rest of text</p>
Then the css for "indented" and "special-f" have two suitable font families, with fallback. But it will fail unless users pick "Publisher" etc on ereader, and some apps and formats simply won't use embedded fonts.