Controlling screen brightness without hardware adjustment buttons

Issue

We have some HP Z1 Workstations, which are all-in-one systems. There are no brightness control buttons for the monitor.

xbacklight (from EPEL) worked fine until RHEL 6.5. Under 6.5, it does nothing, and reports no errors.

xbacklight works on a 6.5 system booted using a 6.4 kernel.

I know this is an EPEL package, but since RH docs reference it, we are requesting help...