I called to enroll in this benefit earlier today. The airlines with discounts of some sort are (in no particular order):
United (includes Lufthansa transatlantic flights marketed by UA and ANA transpacific flights marked by UA),
Aereolineas Argentinas,
Air Europa,
Air New Zealand,
Avianca,
Cathay Pacific,
Copa,
Fiji Airways
Qantas,
Qatar.
I'd be curious what the discount amount is; the terms state that it can vary based on fare class. Note that some of these discounts only apply on certain routes and may not be available for partner flights marketed by the discounted airline.