Is there a way to set ForkLift as the default file viewer, to a degree? PathFinder somehow does this, see http://cocoatech.com/faqs#3, but how does it do this and could that option be set to redirect to ForkLift instead of PathFinder?
Path Finder looks like it's modifying the "NSFileViewer" preference. You can set this manually from Terminal to point to ForkLift (I tried this, and it seems to work):
However, be warned that the Path Finder website lists some applications that don't respect this setting, such as the Dock and Firefox.