Are you keeping it indoors? Is that why you want to wear a diaper if you keep it outside and it poops outside the rain will melt it away or you can hose it down. It will dissolve and fertilize your lawn.
What breed of goose is it? Can you share a picture? I wonder how old it is ? If you give it...
I just went through something similar. I used the information in these two links to help me through it.
https://www.backyardchickens.com/articles/step-by-step-guide-to-assisted-hatching.64660/
Miss Lydia said
Did she say how long he has had this? I really don't know what it could be since she said she massaged it and it went down in size. So doesn't sound like a tumor. I'm not sure if ducks get muscle spasms. Anyone else give any ideas?
I just fwdd ur msg to miss Lydia (duck educator here who can’t get online due to no Internet since hurricane Helene) I’ll let you know if she has any ideas. I’ve never seen or heard of that before.