My understanding is that Greenguard needs to be covered by a thermal barrier like drywall when you install it. Is there a way to use a leftover piece of Greenguard to make an attic access insulation cover safely?
Sure it can. Greenboard IS drywall, but it has a thicker paper shell that's coated with wax to make it water resistant. It's typically used in high-humidty locations such as bathrooms. However, it's not water-proof and shouldn't be used where direct exposure to water is expected (such as a shower stall).