Since no one answered yet, there might not be a one line solution.
All I could think of was a workaround: call /usr/bin/open with the -R option, which will reveal the given file in Finder. Since you want to show the contents of the package, you will have to reveal any file that is inside (not the package itself).
Downsides: won't work on empty package (but then you could just reveal the package), also the Finder window will show a selection on the last item.
NSString *pathToBundle = @"/tmp/test.app";
NSFileManager *fm = [[[NSFileManager alloc] init] autorelease];
// get the last file/directory in the package
// TODO: error handling / empty package
NSString *lastItemInBundle = [[fm contentsOfDirectoryAtPath:pathToBundle error:NULL] lastObject];
// reveal this file in Finder, by using /usr/bin/open
// see -R option in the manpage
NSTask *open = [[[NSTask alloc] init] autorelease];
[open setLaunchPath:@"/usr/bin/open"];
[open setCurrentDirectoryPath:pathToBundle];
[open setArguments:[NSArray arrayWithObjects:@"-R", lastItemInBundle, nil]];
[open launch];