That sounds more like intervention than prevention?
I’m hoping you aren’t really feeding 1/4 gallon MO on a regular basis. Doing so will negatively impact nutrient absorption.
the blockage is usually a long way from the stomach too You can induce a laxative effect by feeding enough Epsom Salts over a period of time, but it’s not going to be laxactive in a single dose, unless it’s a HUGE amount, and you don’t want to do that. There are various magnesium sources that are more likely to cause diarrhea than others, when fed in larger amounts, but not in single or even a couple of doses, unless it’s a huge amount, and that’s not safe