Hi Matthias,
CreateMeta resource with
expand=projects.issuetypes.fields param should return you all possible options. Field options depend only on project and issue type (not on the screen). If issue types are not specified then all possible options for each issue type for the given project are returned.
That is the case for all built-in JIRA field types; However plugins may provide custom field types with may behave differently and returns different set of options for different issues.
Let me know if that helps you.
Best Regards,
Marek