  Using four different suites of cosmological simulations, we generate
synthetic spectra for galaxies with different Lyman continuum escape fractions
(fesc) at redshifts z=7-9, in the rest-frame wavelength range relevant for the
James Webb Space Telescope (JWST) NIRSpec instrument. By investigating the
effects of realistic star formation histories and metallicity distributions on
the EW(Hb)-beta diagram (previously proposed as a tool for identifying galaxies
with very high fesc), we find that neither of these effects are likely to
jeopardize the identification of galaxies with extreme Lyman continuum leakage.
Based on our models, we expect essentially all z=7-9 galaxies that exhibit
rest-frame EW(Hb)< 30 {\AA} to have fesc>0.5. Incorrect assumptions concerning
the ionizing fluxes of stellar populations or the dust properties of z>6
galaxies can in principle bias the selection, but substantial model
deficiencies of this type will at the same time reveal themselves as an offset
between the observed and simulated distribution of z>6 galaxies in the
EW(Hb)-beta diagram. Such offsets would thereby allow JWST/NIRSpec measurements
of these observables to serve as input for further model refinement.
