I am not sure that it works the same way with LH but with PS marketed but OS operated flight I can usually get an OS 'operating' PNR which I can then use for seat selection and special meal orders on aua.com. Try checking your reservation on checkmytrip.com and expand information for the OS-operated but LH-market segment - it should give you the OS number (they may be the same though). If it does give you one see if you can use it for meal orders.
BTW - I ordered Do&Co meals on my recent flights to/from Vienna and they were very good.