Currently, Piwik restricts the naming of plugins using the following regular expression: '/^[a-zA-Z]([a-zA-Z0-9]*)$/D'. I propose that any valid PHP class name be a valid plugin name. According to the PHP documentation, a valid class name is defined as,

The class name can be any valid label, provided it is not a PHP reserved word. A valid class name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: ^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$.

Barring this expansion. I propose that underscore character included as a valid character for plugin names.