I was thinking of doing that but I'm not sure how i'd get a plate on my back. Fo rthe reverse crunch, I tried using a medicine ball and toward the end of the set, accidentally dropped it (nearly over my head)
Made me scared to try with a dumbell.
I've done the reverse crunches on the incline/decline? crunch bench -- angled so feet are lower than head. Don't let your feet touch the ground so you never really release at the bottom.
A woman at my gym does the DB between her feet, but I've never quite noticed how she moves it so it's always working right.
For pushups, you could make them explosive, t pushups, or some other variation that makes it more difficult for you.