Any way to change properties of subcomp based on parent comp?
You can link properties in different compositions by using an expression like this one.

comp("Comp Name").layer("Layer Name")("Property e.g. Transform")("SubProperty e.g. Position")


If the property is more nested you just add another ("sub property") until you reach the property you need, just as you would click through the drop down menus.

Also helpful: VideoCopilot recently released a great free script specializing in pre-comping 3D layers in exactly that way. http://www.videocopilot.net/blog/2014/08/free-script-3d-pre-compose-tutorial/

Edit: Regarding the clarification in the comments about using relative selectors for comps.
Unfortunately not possible per se. You can do it with layers by specifying the index number but a comp doesn't really have a unique parent which makes this rather hard to have a generic selector for comps. If you have a unique procedural naming based on your comp "depth" this might be possible with some advanced expression or maybe script but if you just named them "my cool comp" and "my other cool" comp you can't really make a script for that.