In my paper called "Secrets of the SG Procedures" (http://support.sas.com/resources/papers/proceedings09/324-2009.pdf), I discuss the topic of displaying custom limits. Currently, the bar chart in the SG procedures does not allow you to specify custom limits; but there are a couple of workarounds:
1) Wide needles (discussed in the paper)
2) Use GTL to define a BarChartParm (a parameterized bar chart)
Both options require the data to be pre-summarized. Let me know if you need any details on option #2.