As a single replacement I might suggest weighted decline situps... However, you could always just do weighted ab work and hamstring/glut work that incorporates the lower back. Good mornings come to mind.
It is very difficult to substitute for the Reverse Hyper and truly do what it does.
If you have something you can lean over and grab onto, you could place a dumbbell between your ankles and try that. Otherwise go with lifts like Good AM's or suspended squats.
yay.