Bumblefoot is essentially a normal callus that has gotten a more dense area, like a corn, within the callus. As a Podaitrist, I understand the mechanism because we see it in people too. The dense area of callus acts like a stone taped to the bottom of the foot, and blister like fluid builds up under the callus. If any bacteria get in, that's when you get a large amount of drainage and cheesey like material under the callus that must be drained. I have ducks and can say that they go very quickly from a plain callus to infected. This may be because of their high body temperature.
Ducks, especially Pekins, get thickened areas on the bottom of their feet. If there are areas of very hard build up, you can use Vaseline to soften the callus and encourage sloughing of the callus. There is a product made for callused feet that could be rubbed on the callus, called Kerasal Ointment. Do not use it on skin that is open, but on calluses it is fine. Soaks are best done using Hibiclens which kills all bacteria, viruses, fungus and yeast on the skin and in the wound for a full 24 hours after a 15 minute soak. But 2 soaks a day is best.
Oral antibiotics should also be started. If infection gets into the joint or bone it is very bad. Baytril 10%, twice daily for 5 days, works.