I finally figured it out: MariaDB keeps an "index" file, fortunately ASCII, that you need to edit: mariadb-bin.index located in the binlog directory (or wherever your log_bin_index configuration points at). This file lists all the binlog files with their full paths. This is unfortunate as the paths should be relative to the log_bin configuration setting. Example:
$ head mariadb-bin.index/<binlog path>/mariadb-bin.000001/<binlog path>/mariadb-bin.000002