The end user does not have the capabilities to remap hardware buttons, nor do normal store applications. One can indeed change the default camera app that corresponds to the two-step camera key, but not much else. Conflicts can become an issue. etc. but it is not unprecedented as many older devices can remap keys like the Palm Treo I own.
Well it can be done if the API to capture volume up/down event is available for use to app devs, certainly cannot be implemented by 3rd party for the built-in camera app. This is where Apple wins in functionality
Edit: Looks like volume button events cannot be captured by app devs