AbstractProjectDescriptor.doAutoCompleteLabel has a complex implementation including AutoCompleteSeeder that ought to be made available as an API in Label.
Similarly doCheckLabel should be available as an API.
For use from Workflow (ExecutorStep.DescriptorImpl), LabelValidator has to be generalized to handle any Job (probably meaning the nested class in AbstractProject should be deprecated and it should be moved to a top-level class). This is assuming that there are some meaningful implementations for other job types; until
INFRA-180 is fixed this is tricky to verify but https://github.com/search?type=Code&q=user%3Ajenkinsci+LabelValidator turns up no hits. (SecurityTokensNodeProperty in cloudbees-folders-plus could add one, perhaps.)