There are more than a few layers of old skin there for it to be so big....did the vet take xrays and biopsy ?
I asked as it looks like perhaps a bot fly lava , and only because I saw a program about scientist who discovered he had one and who let it stay on / in him to study it.
Info :
http://entnemdept.ufl.edu/creatures/misc/flies/human_bot_fly.htm , if it is , it appears very advanced , and close to pupation (the maggot crawls out of the hole it's made and drops to the grown where it will want to burrow down to pupate).
My understanding is they produce enzymes that act as an anesthetic and antibiotics to keep the wound from becoming infected by viruses and bacteria.
If it's botfly maggot causing this I'd let nature take it's course if it's not worrying the dragon , and seek out vet treatment when the maggot leaves to ensure the vacated "growth" doesn't become infected.