If no other alternative treatment that's possible is effective, someone who wants to take a risk could try treating with
Tangerine essential oil.
Mixing a drop of Tangerine essential oil in a a little bit of carrier oil (such as coconut oil) to dilute it, & then rubbing some of that on the foot would be the probably the most effective & safest way to use it. Essential oils are EXTREMELY concentrated, so be very conservative on amounts.
Tangerine is supposed to kill some kinds of staph infections, which is the class of infections that usually cause Bumblefoot. I think the tangerine PEEL is what's helpful, so regular tangerine juice wouldn't be very helpful.
https://www.organicfacts.net/health...alth-benefits-of-tangerine-essential-oil.html
http://www.theatlantic.com/health/a...w-antibiotics-might-be-essential-oils/384247/
Citrus can be risky for birds, however.
https://www.backyardchickens.com/t/274883/why-cant-you-feed-your-chicken-citrus
Iron overload disease (extra risky if high-iron pellets, supplements or foods are also fed):
http://www.beautyofbirds.com/ironstoragedisease.html
I don't know if the risks would be quite as high if the bird is absorbing tangerine thru the skin rather than taking orally, but they might be.
I put 2-3 drops of lemon essential oil in my 3 chickens' drinking water a number of times over a year ago, and the two hens seemed to have done fine. The rooster died from a combination of problems, so I don't know if it had a detrimental effect for him or not. He had gout (among other conditions) & I also applied lemon EO undiluted (probably unwise) on his feet.
The below article specifically recommends feeding citrus to chickens, but it doesn't sound like the people involved necessarily know a lot of specific info about birds.
http://www.examiner.com/article/why-feeding-oranges-to-cows-and-chickens-could-save-lifes
Try googling to research more about these issues before deciding to try this treatment out!
Tangerine essential oil is pretty inexpensive. It's $6.56 + shipping at this website--
http://butterflyexpress.net/shopping/index/OS
If anyone decides to experiment with this, please report back on results!