Path Finder has some preferences that might help, like “Set as the default file browser” and you can tell it to take over file:// URLs. That alone might be enough, but I would also recommend going into Quicksilver’s preferences, go to Handlers, and set Path Finder as your File System Browser.
I’m not sure which of these settings affect the Open action, as I changed them all around the same time when I switched (back) to Path Finder last week, but one of them should take care of it.
I’d like to point this out in a more obvious place soon, but for anyone reading, I highly recommend Path Finder for use with Quicksilver. The Finder bugs that were introduced with 10.7 (and still aren’t fixed) are not present in Path Finder. That is, the selection is always reliable, and the Services menu works in column mode. You can also view and change OpenMeta tags if you’re into that.