I can now tell you how it went - flew CPH-MAD on Iberia Express:
Entirely an economy experience.

All catering was based upon the economy class purchase menu where C-class passengers could choose what they wanted for free.
But no tray with a hot meal, salad or dessert - no real steel cutlery or glass-ware or porcelain cups for the coffee. EVERYTHING was plastic.
Service was friendly enough but the experience had absolutely nothing to do with business class.

Will I fly this again - I think not - If an airline offers and charges for C-class the offering should match. Paying C and getting Y is not for me (or anyone else I expect)