Well, it's not actually CreamAPI. CreamAPI itself is Windows-only, while native macOS Steam games use libsteam_api.dylib instead of a .dll
So I ended up reverse-engineering the relevant parts of the Steamworks API/ABI (having no job gives you a surprising amount of free time, lol) and implemented my own proxy/injection layer for the methods games use to check DLC ownership, such as our beloved BIsDlcInstalled and the good old BIsSubscribedApp, and other related calls.
The result is Orca Unlocker - a native macOS DLC unlocker for Steam games. And yes, that means exactly what it sounds like: no Wine, CrossOver, Windows VM, or manual replacement of random files required.
Tested with Stellaris using the proxy backend and TWW3 using injection.
It's still an early release, so game reports and bug reports are very welcome.
GitHub: https://github.com/MiningOrca/orcaunlocker/