There are several ways of customising AdvancedSkeleton to your specific needs.
For custom `selection curves`, you can open the file "asCurveIcons.ma" which contains the icons,
and make any changes, as long as the names remains unchanged.
For modifications of the default values of AdvancedSkeleton, you can use the “run” attribute.
This is a string attribute on the FitSkeleton node (Group|Main|FitSkeleton).
Select the FitSkeleton node, and in the attribute editor, the "run" attribute is in the "Extra Attributes" section
Any mel command entered here, will get called any time the AdvancedSkeleton gets built,
As well as any time the function “Reset Pose” gets used by the animation tools.
For example, if you would like a character to have its spine set to IK by default,
then you could add the following to the run attribute: setAttr "FKIKSpine_M.FKIKBlend" 10;