In the 6.5 release (http://www.daypilot.org/daypilot-pro-6-5.html), it will be possible to specify the displayed hours range independently of the business hours and HeightSpec using DayBeginsHour and DayEndsHour.
You will be able to limit the displayed hours using DayBeginsHour and DayEndsHour and still use HeightSpec="Parent100Pct".
The implementation is already available in the sandbox:
http://www.daypilot.org/sandbox/