Sorry for answering myself. I have RTFMed a bit in the while.
I actually hint this can be simply done with a LUA filter.
I will try to implement it as a self-exercice and report the result here.

On Tuesday, September 21, 2021 at 9:34:26 AM UTC+2 Vin Cent wrote:

> Hi,
>
> I am generating epub3 from latex source.
> I was wondering why footnotes show a backlink to the original text when I 
> display the document in calibre, and why they do not when I display the 
> document on a kobo reader...
>
> ... until I found the backlink is indeed well present. It is "just" not 
> displayed by kobo because the character set (by pandoc, I think) has no 
> rendering on their font.
>
> The backlink character, copied and pasted from the .epub document itself, 
> seems to be "↩︎".
>
> I see two possible tricks here so that the link will appear on kobo :
> 1. Change the character to another one that would be rendered by kobo.
> 2. Change the default font of the document to one that has rendering for 
> that character.
>
> I have tried all available fonts on my device. There are actually three 
> classes of them.
> (Listing them all below, this might be of interest to somebody in the 
> future)
>
> Avenir Next ; Georgia ; Kobo Nickel : these fonts render absolutely 
> nothing for that character, leading the human to believe the backlink does 
> not exist. It is present, clickable, just not rendered.
>
> Amasis ; Caecilia ; Gill Sans ; Malabar ; OpenDislexic : these fonts 
> render "__" for that character. I find it "better" than the first family, 
> but still not great. It is not obvious for a non-tehnical human that this 
> is a backlink to the text.
>
> AR UDJingxihei ; Kobo UD Kakugo ; Kobo Tsukishi Mincho : display oriental 
> character (I think this is chinese / japanese depending on font) for the 
> backlink character.
>
>
> Therefore, I tend to favor the first solution. Do you guys know of a way 
> to customize the "backlink ↩︎" character set by pandoc in epub ?
> Or is there a third approach ?
>
> Thanks,
>
> Vincent
>

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