Apologies for the late response (travel, apocalyptic virus, new machine, other fun…).
The custom composer source in my vault is:
vault_mysql_root_password: password-here
vault_wordpress_sites:
clientsite.com:
admin_password: password-here
env:
db_password: password-here
# Custom Composer Source:
composer_authentications:
- { hostname: satispress.company.com, username: username-here, password: password-here }
Running the manual composer config command results in no error (no feedback at all, in fact).
Running composer config --list does indeed display the config details for this custom composer source.
After the above steps, I also went back to project root and attempted another trellis provision development run: Still stalling at wordpress-install : Setup composer authentications.