The leprous has a small chance to drop either the belt or mask (I have gotten each one several times across the two years in zone).
I have gotten the items off a leprous scarab that spawn from the mobs that roam the hallways between the table room in the far sw area (that leads to chef/repairer, Froggy, and armsman) and the bartender room. Basically if you are killing the chef and bartender, you will encounter the scarab or its phs. The scarab/ph can be a frog, golem, or scarab (pox or leprous). I've only seen the mask and girdle when the mob is a leprous scarab though.