I don't think you can get around this sort of thing by booking under codeshare numbers - as indicated above, NZ/UA/BD/&c 'M' class on AC metal will simply get translated into whatever the equivalent fare class is in AC's scheme.

E.g. 'A' means 'discounted F' on LH but 'cheapo Y' on AC. If you were to book a cheapo flight on LH metal under an AC codeshare # in AC's A class, that wouldn't mean you get to sit in F !

The complimentary food for Y M U pax is not much to get excited about in any case. There are no 'meals' as such in Y within North America , there is merely a selection of BOB sandwiches and junk food. As a YMU pax your sole distinguishing privilege is that you get 1 sandwich and 1 candy bar/packet of crisps out of the trolley for free. You still have to pay for wine/spirits ; non-alcoholic drinks are free for everybody.