This is a good question; unfortunately we can not provide a definitive answer. The Department of Defense Government Charge Card Guidebook for Establishing and Managing Purchase, Travel, and Fuel Card Programs and the Army GPC Operating Procedures do not expressly prohibit the use of the GPC (or appropriated funds) for this purpose. Additionally, neither the GAO Red Book or DoD Financial Management Regulation expressly prohibit the use of appropriated funds for this (portable bathroom) purpose.
But their is danger. Therefore we suggest you get with your local or agency A/OPC and fiscal law attorney and get their advice and guidance prior to making any decision.
Again, we can not provide a definitive answer because it requires us to know many more details such as location, attendees, etc.