Not sure if relevant to anyone, but I've setup mine this way on windows with Maya 2023, and it seem to work .
Inside this folder (if you don't have a \modules folder, create one):
C:\Users\USERNAME\Documents\maya\modules
i have created a file :
custom_maya_setup.mod
inside this file I saved these paths to the .json file for each of the extra compounds modules I wanted to try:
BIFROST_LIB_CONFIG_FILES += D:/FOLDER/ANOTHER_FOLDER/bifrost_compounds/MJCG_compounds_2_1_0/bifrost_lib_config.json
BIFROST_LIB_CONFIG_FILES += D:/FOLDER/ANOTHER_FOLDER/bifrost_compounds/rebel_pack_0_4_1/bifrost_lib_config.json
Restart and running Maya now, it picks up the .mod file, and it seems to load the custom bifrost compounds paths as well.
This way, if I want to add another compound or get rid of an old one, I can do that by modifying 1 .mod file and I can keep all the custom compounds in separated folders with a nice recognizable name.