As tool user (e.g. local-tusc), I can connect to the tools-db server using the replica.my.cnf credentials, but cannot connect to the "mysql" database (as the "sql" command would), nor can I create tusc__something databases there.
Someone wrote on https://wikitech.wikimedia.org/wiki/User:Magnus_Manske/Migrating_from_toolserver that this requires a .my.cnf file for the tool. Some of my tools (e.g. flickr2commons) have that, others (e.g. tusc) don't.