Comment 5 for bug 775113

If it is impossible or undesirable to use strftime() instead of g_date_time_format() in the rendering, then this bug is a documentation issue. The summary for the custom-time-format field in the dconf-editor clearly states that strftime() is used.
"Summary: The format string passed to strftime"

The description should be revised as well:
"Description: The format of the time and/or date that is visible on the panel when using
     the indicator. For most users this will be a set of predefined values as
     determined by the configuration utility, but advanced users can change it
     to anything strftime can accept. Look at the man page on strftime for
     more information."

But I hope it is possible to change the indicator-datetime code to use strftime(), simply because that function does the job already and no upstream fix would be required.