@madamczak - I see API listed as other API “Web Resource context” *https://developer.atlassian.com/server/bitbucket/reference/web-resource-contexts/) which let you render custom data through css/javascript to the core pages, and the context available for file/diff view - “bitbucket.page.repository.fileContent”. I hope this will help in my use case.
I tried though adding the web-resource only with the above context and the javascript which just outputs using document.write but I get a weird view on file/diff file of any file from the repository, I have raised that under another issue.