For uncontrolled diarrhea style messing you could use Epsom salt. This will result in a messy diaper without any doubt. For me it takes about an hour to work, so when you are really tired, and take it just before sleeping, it might do the trick of messing in your sleep.
Be sure you have the Epsom salt suitable for consumption, and to prevent dehydration, make sure you drink enough.
Worked for me once!
Good luck