The screensaver unlock dialog was made themeable a few years ago but it never really worked well and is currently unsupported. The code has been removed from upstream. It would be advantageous to remove it for RHEL 6 too since it adds unnecessary complexity, requires disk i/o every time the screen is unlocked, is potentially fragile, etc.
This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux major release. Product Management has requested further
review of this request by Red Hat Engineering, for potential inclusion in a Red
Hat Enterprise Linux Major release. This request is not yet committed for
Red Hat Enterprise Linux 6.0 is now available and should resolve
the problem described in this bug report. This report is therefore being closed
with a resolution of CURRENTRELEASE. You may reopen this bug report if the
solution does not work for you.