As the user @DFriend stated in this question:
Extending CI_Log is not going to work if you need to access other libraries. The reason is CI_Log is created long before $CI is created so no "instance" is available for &get_instance() to return.
$this->load doesn't work because $this is not a controller ($this and $CI point to the same object) and the class load ('CI_Loader') hasn't been created yet either.
The way I was proceeding doesn't look like the way to go. Since extending the CI_Log class has clear limitations as the modification of the core class itself.
For the moment and since it is a temporal modification I will proceed overriding the CI_Log class and using plain PHP to complete the task.