I am not aware of any changes to the lookup of the the specifics files.
And everything you describe looks good.
Can you try to use the QML_IMPORT_PATH variable of qmake instead of the QML2_IMPORT_PATH environment variable?
6.Create a designer folder
7.Add following to designer.pri
QML_FILES += $$PWD/myextensionplugin.metainfo
8.Build and install
9.Generate qmltypes: [You can also do this during qmake]
qmlplugindump mycompany.qmlcomponents1.0 > plugins.qmltypes
10.The controls will be available in the qml designer.