Hey,

It seems that, when overriding an admin_setting_configmulticheckbox in the config.php it does not make the setting readonly.

i noticed this when setting this:

$CFG->forced_plugin_settings = array(

  'block_myoverview'  => array('layouts' => 'list,summary')

); 

It shows the the "Defined in config.php" box and also does not save any changes, so the forced setting is working as in expected.
It just does not seem consistent behaviour.

The respective class in the adminlib does not even mention readonly so is the intended?

Seems like it would be easy to fix
with a simple edit in the mustache and the context of of the multi checkbox class.