She has lost her outer layer of skin (epidermis) due to maggot damage.
What you are seeing is probably a fat pad behind the thicker inner layer (dermis). Could possibly be a mass (salpingitis?), tumor or indication of a reproductive disorder - often hens with reproductive disorders have distended abdomens like that- it's just that skin and feathers hide the bulge..
Hard to know IF that will go down. All you can due is continue treating her like you are.
I would not give Aspirin - you mention she seems comfortable, so I see no benefit of it.
Honey can be used on wounds, raw honey works well, it can be messy like ointments - but yes - you would want to keep the patient inside where Flies can't get on the wound. Flies getting on a wound, compromised tissue or on irritated skin with poop is generally how Fly Strike begins.
You can try keeping it moist with triple antibiotic ointment instead of just the Vetericyn spray to see if that helps.