E.A.
Not interoception, but one of my sons had encopresis, in which the child cannot control his bowel movements because the nerves in the lower colon are damaged (we suspect due to a bad bout of constipation as an infant). We used behavior modification (in addition to giving him Miralax daily) and eventually (it took a few years, which is common for these kids) he learned the cues his body was sending to him to let him know it was time to go. Basically, 10 -20 min after each meal or snack he sat on the toilet for about 5 minutes. This retrains the colon to evacuate after a meal in preparation for the new food it's about to digest.
My son could not feel the need to go, so the poop would build up and come out in a big "explosion", alternately leaking watery fluid around the mass. Sometimes this happened daily. He could not feel it, he could not smell it, and he often tried to hide it. I know it's probably not the same thing your child is dealing with, but I suggest the regular bathroom sits.
And I'm sure this goes without saying, but only positive reinforcement. Good luck, let me know if I can help more.