  Motivated by discovering strongly-lensed supernovae, gravitational waves, and
kilonovae in the 2020s, we investigate whether to build a watch-list of
clusters based on observed cluster properties (i.e. lens-plane selection) or on
the detectability of strongly-lensed background galaxies (i.e. source-plane
selection). First, we estimate the fraction of high-redshift transient
progenitors that reside in galaxies that are themselves too faint to be
detected as being strongly-lensed. We find $\sim15-50$ per cent of transient
progenitors reside in $z = 1-2$ galaxies too faint to be detected in surveys
that reach ${\rm AB}\simeq23$, such as the Dark Energy Survey. This falls to
$\ls10$ per cent at depths that will be probed by early data releases of LSST
(${\rm AB}\simeq25$). Second, we estimate a conservative lower limit on the
fraction of strong lensing clusters that will be missed by magnitude limited
searches for multiply-imaged galaxies and giant arcs due to the faintness of
such images. We find that DES-like surveys will miss $\sim75$ per cent of
$10^{15}$M$_\odot$ strong lensing clusters, rising to $\sim100$ per cent of
$10^{14}$M$_\odot$ clusters. Deeper surveys, such as LSST, will miss $\sim40$
per cent at $10^{15}$M$_\odot$, and $\sim95$ per cent at $10^{14}$M$_\odot$.
Our results motivate building a cluster watch-list for strongly-lensed
transients that includes those found by lens-plane selection.
