Thank you both for your replies; I've changed the .bch file.
Analyzing the parent job's detailed logs, I'm seeing something I can't explain: a 10-second delay between databases' backups being initiated from SQL:
08/24/2017 17:32:04 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 165
08/24/2017 17:32:14 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 166
08/24/2017 17:32:24 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 167
08/24/2017 17:32:34 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 168
08/24/2017 17:32:44 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 169
08/24/2017 17:32:54 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 170
08/24/2017 17:33:04 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 171
08/24/2017 17:33:14 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 172
08/24/2017 17:33:24 - Info dbclient (pid=3564) Batch = BACKUP003_WKCCHPRDSQLP1V_TRLOGL_DAILY.bch, Op# = 173
Is this something built-in to MSSQL or specified in NBU somewhere?