  The decay of the narrow resonance $\overline{B}{}_{s2}^{*0}\!\rightarrow B^-
K^+$ can be used to determine the $B^-$ momentum in partially reconstructed
decays without any assumptions on the decay products of the $B^-$ meson. This
technique is employed for the first time to distinguish contributions from
$D^0$, $D^{*0}$, and higher-mass charmed states ($D^{**0}$) in semileptonic
$B^-$ decays by using the missing-mass distribution. The measurement is
performed using a data sample corresponding to an integrated luminosity of 3.0
fb${}^{-1}$ collected with the LHCb detector in $pp$ collisions at
center-of-mass energies of 7 and 8 TeV. The resulting branching fractions
relative to the inclusive $B^- \!\rightarrow D^0 X \mu^- \overline{\nu}_\mu$
are $f_{D^0} = \mathcal{B}( B^- \rightarrow D^0\mu^-\overline{\nu}_\mu
)/\mathcal{B}( B^- \rightarrow D^0 X \mu^- \overline{\nu}_\mu ) = 0.25 \pm
0.06$, $f_{D^{**0}} = \mathcal{B}( B^- \rightarrow ( D^{**0} \rightarrow D^0
X)\mu^-\overline{\nu}_\mu )/\mathcal{B}( B^- \rightarrow D^0 X \mu^-
\overline{\nu}_\mu ) = 0.21 \pm 0.07$, with $f_{D^{*0}} = 1 - f_{D^0} -
f_{D^{**0}}$ making up the remainder.
