I had two megacolon rabbits. For both I was able to keep it managed very well with a strict diet. What worked for my rabbits(but may not be the best diet for all megacolon rabbits) is a no pellet, no sugary starchy foods diet. Instead they were free fed grass hay, and select leafy greens and forage.
The quality of the hay made a distinct difference. If it was too leafy with not enough stalky stems, one bun would start to get very large moist fecal logs. The best type of hay for him was a medium coarse grass hay(I used timothy) with plenty of stalky stems to provide the indigestible fiber that he needed to help stimulate gut function, along with some leafy stems for the needed nutrients. Then there were a few leafy greens he could have like some limited dark leaf lettuces and cilantro. He also could have some dried willow leaves and plantain. And I provided a salt lick since they weren't getting their sodium intake from pellets.
The other bun was most sensitive to carbs and sugars, including the ones in pellets. He was having reoccurring bouts with stasis until I figured out the pellets were causing it. So once he was off of pellets and carb/sugars, he had no further problems and his fecal poop stopped being so irregularly shaped and was almost normal looking. Their strict diets kept both buns fecal poop somewhat normal, particularly for a megacolon bun.
Fine tuning the diet was a matter of trial and error. Like getting a batch of hay that was too leafy and seeing the resulting changes in their poop. Or trying a veggie and seeing moist or mushy poops. Just keep in mind that experimenting with the diet can put megacolon buns at risk. So if any thing new is tried I always did it with extremely small amounts and very slowly and carefully monitoring. Once I was able to get them stabilized, I stuck with that diet and didn't change anything.
Both rabbits did really well, maintained a healthy weight, and had minimal health issues(other than the megacolon). One rabbit did well for two years on this diet, until he passed from an unrelated medical problem. The other rabbit lived to be 5 when he quickly declined and passed from the complications of the disease. So the right diet can help limit complications of the disease like GI stasis, and can help prolong their life.
These are some good links on megacolon in rabbits if you haven't already seen them.
https://www.vgr1.com/megacolon/
http://www.medirabbit.com/EN/GI_diseases/Mega/mega_en.htm
https://rabbitwelfare.co.uk/wp-content/uploads/2013/05/RO-Spring-17-Megacolon.pdf
