Some say citrus is supposed to be unsafe for birds, but if you want to experiment--
You could try 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.
Citrus can be risky for birds, however.
Iron overload disease (extra risky if high-iron pellets, supplements or foods are also fed):
I don't know if the risks would be as high if the bird is absorbing tangerine thru the skin rather than taking orally.
This 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.
Try googling to research more about this before trying it out.
Tangerine essential oil is pretty inexpensive. It's $6.56 + shipping at this website-- http://butterflyexpress.net/shopping/index/OS