Through the Oley Foundation newsletter, I learned that the seller, Retrofied, at etsy.com has experience customizing the backpack pictured above for use with enteral feeding pumps! What an amazing and fashionable alternative to the pricey and somewhat unattractive standard choices through med suppliers. You can even choose your own fabrics and communicate any other customizations you might require.
Make sure you have a loop inside (velcro?) for hanging the bag of formula and an opening cut in a lower side/corner to allow the tubing to exit. Consider another loop for securing excess tubing, and additional pockets for storing other supplies or personal items.
What a lovely way to stay portable!