It doesn't always happen right away, it can happen when you're being transported or back at the morgue. So CTB in a specific position would only help temporarily, if at all.
I'm not an expert on death poo, but I can think of a few options to consider:
- Fasting might help, but on the flip side it can cause diarrhea.
- An enema might help, but it would have to be a pretty hefty enema which would take a lot of effort and be uncomfortable.
- If your timing to CTB was flexible, you could just wait until you take a shit regularly then CTB.
- A butt plug could seal things up tightly, and there are even some specifically made for fecal incontinence. The size and shape is important or it will just shoot out when the shit puts pressure on it. Whether being found with a butt plug is more or less embarrassing than shitting yourself is a matter of personal preference. For some kinky people it could be a way of taking kink to the grave.
A middle ground approach could be to just eat light and try to not worry about it too much. The professionals who work with dead bodies are used to these things, so they'll have procedures to handle it safely.