  A black-hole neutron-star binary merger can lead to an electromagnetic
counterpart called a kilonova if the neutron star is disrupted prior to merger.
The observability of a kilonova depends on the amount of neutron star ejecta,
which is sensitive to the aligned component of the black hole spin. We explore
the dependence of the ejected mass on two main mechanisms that provide high
black hole spin in isolated stellar binaries. When the black hole inherits a
high spin from a Wolf-Rayet star that was born with least $\sim$ 10% of its
breakup spin under weak stellar core-envelope coupling, relevant for all
formation pathways, the median of the ejected mass is $\gtrsim$ $10^{-2}$
M$_{\odot}$. Though only possible for certain formation pathways, similar
ejected mass results when the black hole accretes $\gtrsim$ 20% of its
companion's envelope to gain a high spin. Together, these signatures suggest
that a population analysis of black-hole neutron-star binary mergers with
observed kilonovae may help distinguish between mechanisms for spin and
possible formation pathways. We show that these kilonovae will be difficult to
detect with current capabilities, but that future facilities, such as the Vera
Rubin Observatory, can do so even if the aligned dimensionless spin of the
black hole is as low as $\sim$ 0.2. Our model predicts kilonovae as bright as
$M_i$ $\sim$ -14.5 for an aligned black hole spin of $\sim$ 0.9 and mass ratio
Q = 3.6.
