Set ForkLift as default file viewer
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):

defaults write -g NSFileViewer -string com.binarynights.ForkLift2


(The -g sets this preference globally for all applications.)

However, be warned that the Path Finder website lists some applications that don't respect this setting, such as the Dock and Firefox.