How to generate fake Boxes metadata in custom gstreamer plugin with pyds?

Yes. PGIE detect objects and generate object meta with bbox information.SGIE get the object meta and crop the object images from video and do the inference.

There is sample plugin gst-dsexample source code in /opt/nvidia/deepstream/deepstream/sources/gst-pluins/gst-dsexample. The code of generating object meta is in attach_metadata_full_frame() function.

Please read the document and source code carefully and make sure you are already familiar with gstreamer.
https://docs.nvidia.com/metropolis/deepstream/dev-guide/text/DS_plugin_metadata.html
https://docs.nvidia.com/metropolis/deepstream/dev-guide/text/DS_plugin_gst-nvinfer.html