Properties P and PR show a dropdown list in the properties sheet of the designer, but only PR let the user change the value. Here are some excerpts from the toy project, which is attached.

Q_PROPERTY(E P READ getP WRITE setP)
Q_PROPERTY(ER PR READ getPR WRITE setPR RESET resetPR)

Q_ENUMS for E and ER are set. The corresponding plugin registers a property sheet extension:

QExtensionManager* manager = i_formEditor->extensionManager();
if(!manager) {
std::cerr << "No extension manager." << std::endl;
return;
}
manager->registerExtensions(new EnumEditorFactory(manager),
Q_TYPEID(QDesignerPropertySheetExtension));

The constructor of the editor created by the factory temporarily unregisters:

i_manager->unregisterExtensions(i_factory,
Q_TYPEID(QDesignerPropertySheetExtension));
m_defaultEditor = qt_extension<QDesignerPropertySheetExtension*>
(i_manager, m_owner);
i_manager->registerExtensions(i_factory,
Q_TYPEID(QDesignerPropertySheetExtension));

The library is moved to the designer folder of the "Qt Creator.app" and links are adopted with install_name_tool. The rest looks to me uncritical.