Anway, good news! I'll polish the defcustom patch a little bit and resubmit in the next days.
Okay here are two patches. The first one is "ready" and works for my usecase.
The second is Clément's idea but seems to be incomplete, the two SVGs being the same, and I'm not sure how to modify it (it displays fine but makes an error when we try to change the sorting order). It can serve as a base for later modifications, personnally I'd be satisfied with just the first patch.
Please let me know if I should modify something else, like for example the tabulated-list group definition: should tabulated-list belong to other(s) group(s)?
Kind regards,
Philippe
0001-Allow-customization-of-sort-indicators-in-tabulated-.patch