Updating Composer autoload files without `composer update`?

I'm working on a project which has a lot of dependencies which I personally control. Whenever the namespace mapping changes in a requirement, I need to run composer update to refresh the Composer autoload file so the namespace mappings can be picked up.

However if I'm making changes in the vendor/ directory directly, there's no need to check out the latest versions through composer update -- all I need is to refresh the autoload file. For a project with dozens of requirements hosted on GitHub, composer update can take minutes.

Is there any way to get composer to refresh the autoload files without having to run composer update?

composer install and/or composer dump-autoload would do the same. (dump-autoload documentation)

Could you modify vendor/composer/installed.json and then run composer dump-autoload