Hi Mathias,
I'm able to see the OneDrive availability status (online/offline icons) after simply enabling the folder options mentioned above. No coding needed. See screenshot.
Compare my MC left-side panel which shows my OneDrive subfolders (green icon overlay indicating all files in sync and available offline) VS.
the MC right-side panel which shows my C: root (no green icons, since these files are not synced to the cloud).
MC also displays correctly for DropBox folders (similar to OneDrive, but not shown in the screenshot).
I do not know how MC is determining the cloud status, perhaps the OS is supplying the info to MC when it reads the extended folder attributes? I'm on Windows 10 64-bit home edition. My OneDrive is version 2018 (build 18.222.1104.0007)
Perhaps enabling the file explorer option above will help someone else see their cloud folder status without additional coding efforts.
Cheers!
Anti-Bacterial