Can Quicksilver be configured to open folders in new Finder tabs?
For folders, as with any other type of file, Quicksilver just asks OS X to open the file. The OS decides what to do from there.

It’s the same as typing open /path/to/folder in Terminal. So, if you can get that to open folders in a new tab, Quicksilver should start doing it too.

(I think Finder would need a preference to control this behavior, and I don’t see one.)