I decided not to deviate from pk_ terminology so that the new code doesn't look too different from the other code in the file. Although, I guess we can at least call the main function ri_ReferencedKeyExists() instead of ri_PrimaryKeyExists(), so I've changed that.
I agree with leaving the existing terminology where it is for this patch. Changing the function name is probably enough to alert the reader that the things that are called pks may not be precisely that.