I'd prefer this symbol:
↩ (U+21A9)
Since screen readers might read the name of Unicode character (=leftwards arrow with hook), an
aria-hidden attribute
might be useful. So a slightly over-engineered example might look like this:
Code:
<a epub:type="backlink" role="doc-backlink" href="#ref1" aria-label="Back to footnote reference 1">
<span aria-hidden="true">↩</span>
</a>
However, I don't know how well
aria-hidden="true" is supported. Maybe you could test it with VoiceOver on your Mac.