I'm thinking a solution should exist already, but I'm unable to find it... so, how could I highlight citations to a work a specific author contributed to?
It's a bit like printing the name of a certain author in bold in the bibliography, but this doesn't work in citations where the author one wants to highlight is hidden in the "et al."
For highlighting an author in the bibliography, I use this solution with the \boldname command.
The highlighted author's name might have non-standard characters, so I use a silly example author, Därwin. The behaviour I want would be like this:

Where to start? Add a keyword and change all cite commands by calling \renewbibmacro*{cite}{...}? And what to put there ?
Here's a MWE, I haven't really done anything, but at least it should show the options I'm using (for the sake of conciseness, I left the \boldname definition out, but I could add it if it is relevant).
\documentclass{article}
\usepackage[latin9]{inputenc}
\usepackage[style=authoryear-comp,natbib=true,%
minbibnames=15,%
maxbibnames=15,%
backend=biber]{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Book{Darwin.84,
author = {C. D\"{a}rwin},
title = {The different forms of flowers on plants of the same species},
publisher = {John Murray},
address = {London},
edition = 2,
year = 1884}
@ARTICLE{Davies.pnas04,
AUTHOR = {Davies, T. J. AND Barraclough, T. G. AND Chase, M. W. AND Soltis, P. S. AND Soltis, D. E. AND D\"{a}rwin, C.},
TITLE = {D\"{a}rwin's abominable mystery: {Insights} from a supertree of the angiosperms.},
JOURNAL = {Proc. Natl. Acad. Sci. U.S.A.},
VOLUME = {101},
PAGES = {1904--1909},
YEAR = {2004},
MONTH = Feb,
NUMBER = {7}
}
@ARTICLE{Vamosi.el10,
AUTHOR = {Vamosi, J. C. AND Vamosi, S. M.},
TITLE = {Key innovations within a geographical context in flowering plants: towards resolving {D\"{a}rwin}'s abominable mystery.},
JOURNAL = {Ecol. Lett.},
VOLUME = {13},
PAGES = {1270--1279},
YEAR = {2010},
MONTH = Oct,
NUMBER = {10}
}
\end{filecontents}
\addbibresource{\jobname.bib}
\makeatother
\begin{document}
\citet{Darwin.84} a souvent été cité, par exemple par
\citet{Davies.pnas04,Vamosi.el10}.
\printbibliography
\end{document}