  We study scalar field configurations around Kerr black holes with a
time-independent energy-momentum tensor. These stationary `scalar clouds',
confined near the black hole (BH) by their own mass or a mirror at fixed
radius, exist at the threshold for energy extraction via superradiance.
Motivated by the electromagnetic Blandford-Znajek (BZ) mechanism, we explore
whether scalar clouds could serve as a proxy for the force-free magnetosphere
in the BZ process. We find that a stationary energy-extracting scalar cloud
solution exists when the reflecting mirror is replaced by a semi-permeable
surface which allows the cloud to radiate some energy to infinity while
maintaining self-sustained superradiance. The radial energy flux displays the
same behaviour for rapidly rotating holes as magnetohydrodynamic simulations
predict for the BZ mechanism.
