I'm not sure that the driver can affect subtitles already on the device, aside from from overriding it, but could be wrong.
I use user-managed #subtitle (long-text, interpreted as short text - yes, the contradiction there is a bit amusing) column that I put into the KoboTouch(Extended) settings.
EDIT: This template takes the value from #subtitle and titlecases it. If there is none, it returns a blank value to override other stuff like redundant series information.
Code:
program: if $#subtitle then titlecase($#subtitle) else '' fi