Hello,
I’ve discovered that it's possible to compile code originally written for PlatformIO or the alternative HAL ch32v003fun directly in the native MounRiver Studio environment without any issues.
There's no need to modify anything—just create a new project in MRS, delete a few files, and replace them.
The exact steps and a specific example using ch32v003fun can be found on my GitHub:
https://github.com/lennox-13/ch32v003fun_MounRiverStudioEverything works, including debugging and flashing via WCHLink-E.