Ian,
If you provide more detail on the codeshare airline and the operating airline, I'm sure someone will have an answer for your case.
Just two examples:
- Back when CO was member of Skyteam, in GVA, they would not provide lounge access to Elite+ members because they were using a contract lounge (although as a FB member, I could then go to the KLM lounge instead).
- With the AF codeshare on QF, I had no trouble getting lounge access in SYD at the lounge operated by QF.
Cheers,
GenevaFlyer