Loading pg_hba.conf during SIGHUP in the backends will solve the problem of displaying the data which is not yet loaded. This change may produce a warning if it fails to load pg_hba.conf in the backends.
This seems like the right strategy to me. It parallels pg_settings and postgresql.conf which means one less surprising quirk for future developers.
An idle thought, in the long-term it seems like it would be better to have postmaster have some shared memory where it dumps out a config data structure that backends can all see. That might help with the race-conditions we have now when reloading config data where different backends could end up with different interpretations of the same config or even see different configs if the files are being modified concurrently. But I think that would have to be done very carefully so the postmaster doesn't sacrifice any reliability.