I am thinking umbilical cord as well. If it was intestines, I would think it would grow into the foot/leg a little since the intestines would be growing for a long part of the chicken's development in the egg. Also, intestines are not usually just dumped in the abdominal cavity like an overturned bag of yarn to swing about - they are encased within a membrane. Also, I think the digestive tract forms before the limbs are completed... but the yolk sac shares the space inside the egg with the completed limbs as the chick absorbs it through the umbilicus and into the navel. I'm thinking horses here, not zebras.
If you haven't already, you may want to cut the cord. If it's digestive tissue, the chick will probably die, but most likely it would have died anyway. If it's umbilical, you can cut it close to the navel as you can, treat the area with a good antibiotic a few times a day (my favorite for chick-navel-issues is powdered sulfur aka flowers of sulfur b/c it is drying, but even neosporn will do). If you don't cut it, the cord might just dry up and fall away, but make sure you put some antibiotic of some kind on the navel, since it's basically just an open wound that goes straight to the major organs.
So glad you freed his foot, and wishing all the best. Keep us posted.