In the default2 config, the Maya launcher is controlled via the Software entity, as are most of the software we integrate with. So as the software is configured in Shotgun, we have two software generic launch app settings blocks:
(One is specific to the tk-shotgun engine though)
So you would add the hook definition to those two blocks.
Then as you mention, your before_app_launch.py hook needs to now include the engine_name parameter, as this hook will be called for all software. You can use this value to work out which software is launching.
That should hopefully do it.