Currently if you have a relative path to an Item from a ModifiableTopLevelItemGroup you need to use Jenkins to get the actual item (Jenkins.getInstance().getItemByFullName(ModifiableTopLevelItemGroup.getFullName() + "/" + itemRelativePath))

It would be handy to do this from the ModifiableTopLevelItemGroup

e.g. ModifiableTopLevelItemGroup.getItemByFullName(path) which would take a relative subpath (would not allow references outside of the folder with ...