I don't have TTSClock, but does it use localtime()? If so, you should be able to use a real timezone in your @EXEC line. I don't think that Hijack uses a shell to interpret the command line, so you'd need to do that explicitly:

;@EXEC_ONCE /bin/sh -c "TZ=CST6CDT exec /drive0/var/ttsclock"

That would save you having to edit config.ini for daylight savings (and save you embarrassment when it's wrong twice a year).

The other possibility is to hack on TTS Clock to accept a timezone name as an alternative to a numerical offset.