As promised, I add the Butt bone to the "OpenSim Jiggle Rig" the file can be downloaded from https://github.com/makehumancommunity/c ... aster/rigs
The file includes the the Pec bones as well. Drop these rigs in /makehuman/v1/data/rigs and your good to go.
NOTE: This rig is not weighted but can easily done in blender.