Can I register an additional namespace to point to the modules folder
Splitting my comment over there into a separate answer then ;)

You can, yes, see system_register() and drupal_classloader_register(). Those helper functions hardcode "Drupal\" as they are all about modules, but you could get the class_loader service directly.

As @Clive said there, I would not recommend doing it for your own classes, there isn't really a reason to do so, unless you're including an external library.

External libraries is a different story, that can be useful. As already commented in How to autoload classes from a 3rd party PHP library in Drupal 8, if you do not want to depend on Composer Manager, then you can implement your own autoloader service and register them either by loading a composer-style autoload.php, or if you have it available in a different way, by directly registering it in the cloassloader service.