I'm relatively new to expressions, but what I really need is to figure out how I can get an expression control (ie. slider) to give me the option to control the weight of influence between the selected layer, and a parent layer of my choosing.
This would be for a character rig, in which I'd like to be able to animate the hand controls freely at times, but at other times have them completely parented to the waist when they no longer need to move independently.
This would be complicated to do through expressions, see my explanation below. For a quicker fix, I know there is a script out there that gives you influence on parents, called Good Parents: http://aescripts.com/good-parents/
Explanation why doing it through expressions is complicated:
Simulating all effects that 'parenting' has is complicated since parenting achieves many different effects at once. Not just a link in rotation, position, scale but also relative position, rotation, etc, PLUS the rotation / scale / position of the child layer itself.
To achieve the control you're looking for, you would need to program an expression that does the exact same as parenting does, and then have a slider (or checkbox) to interpolate (or switch) between the simulated parent-influenced value and the non-influenced value.