As far as my knowledge goes, here is how it's done: eZ Publish module operations are defined in a file called operation_definition.php. Currently there are 2, one for content, and one for shop (kernel/content/operation_definition.php, kernel/shop/operation_definition.php).

These files define operations. I'm not exactly sure how operations are different from views, but the definition is quite similar, except for the complexity. Operations have an additionnal key, 'body', which defines how the operation runs. body is defined by a sequence of methods AND triggers (there we are). For instance, the body for content/read is:
- pre_read (trigger)
- fetch-object (method)

As far as I undertand it, what you have to do to add a custom trigger is replace the view you want to alter by an operation, and add the trigger. I hope it helps :)