I did not find anything from the Mercurial hook documentation explaining how to get properties of the repository's hgrc file from a custom python hook.
As the documentation states that we have access to the full Mercurial API from hooks, I suppose this should be possible. But how?
Moreover I could not find a precise documentation on what are the objects (ui, repos, ...) passed in argument to the Mercurial hook.
Ok I find out the answer by myself.
This page gives some hints about the argument passed to the hook functions: https://www.mercurial-scm.org/wiki/MercurialApi#Reading_configuration_files
ui.config
functions permit to read Mercurial configuration files, giving the section and property name.