D'oh! Right after I posted this, I went back to take another troll
through the test just in case and discovered the answer to my
problem. It's in Test #27 for anyone else interested, and basically
looks like this for the example above:
var isDirectory = new outArgument;
// Result is something like exists == 1, isDirectory == 0
I haven't tried this with something like an NSError** argument, but
hopefully that will function the same.