Instead of AuthId, you could use the CrsId in this way:
from qgis.gui import QgsGenericProjectionSelector
projSelector = QgsGenericProjectionSelector()
projSelector.exec_()
crsId = projSelector.selectedCrsId()
target_crs = QgsCoordinateReferenceSystem()
target_crs.createFromId( crsId, QgsCoordinateReferenceSystem.InternalCrsId )
iface.mapCanvas().setDestinationCrs( target_crs )