thanks, i got it working.
i noticed the -g for debugging symbols, which was missing in my debug build. When I added that the resources started working, but in release it still didn't work. So i thought it's probably to do with symbol visibility. I modified my IPlug_include_in_plugin_hdr.h to define the EXPORT macro differently for Mingw and this seems to have done the trick.
this is probably not an issue with the stock IPlug, but some change I did in the past to my repo meant it was needed
#define EXPORT __attribute__ ((visibility("default")))
#define EXPORT __declspec(dllexport)