Alter the views exposed filter output of term reference tree

Probably this is not related to this module's functionality but I'm sure it's a common use case when using this module.

I enabled this widget on a custom field added to the user profile.

My taxonomy structure is 1 level deep:

-Root term 1
-- child 1
-- child 2
-Root term 2
-- child 1
-- child 2

I want to prohibit the user to select root level terms. This works perfectly by selecting Leaves Only

Now I want to use these terms as a filter field in a view (dropdown select field). But in this filter I don't want the user to be able to select the root levels. I am aware that this functionality is not implemented yet http://drupal.org/node/1301764.

But does anybody know which (views) function to use to add optgroup functionality to this output?

Thanks in advance