It can be difficult to tell constipation from a partial bowel obstruction. In both cases drinking adequate water may help prevent or clear the event, but this can vary a lot. Soluble fiber can help prevent constipation (as long as there’s adequate water in the gut!), but it’s not going to do much for an obstruction. Laxatives of various types may help with constipation, after you’ve already made bricks, but they do tend to be targeted at the colon. Many will still work for a J-poucher, though. If the constipation is in (or near) the J-pouch a tap water enema may help clear it. You can use a Fleet enema bottle and replace the contents with warm water. Be careful of high-volume enemas, since you don’t have a high-volume colon.
The key ingredient in every case is water, though you don’t want to force it into a blocked gut.