Thanks for your comments.
I decided to go ahead and write the unit tests for that layer, both because I knew what not writing them would be like, and I wanted to play with wrapping/mocking a system service.
I also decided - as some of you commented - to do the right thing and encapsulate it into a class. That would have happened long ago, but though I've written it several times, I don't think I've ever duplicated it within a single codebase - and the codebases where I did write it are pretty disparate. Now, I have something where I could at least move the source file around...
Writing tests for this was a bit weird, because in some sense what I needed to do was figure out what the system behavior was, break that down, write a test against my objects, and then write mocks that allowed me to simulate the underlying behavior.
So, for example, I created a test to enumerate a single file in a single directory, wrote a wrapper around DirectoryInfo, and then created a mock on that object so I could write GetFiles() to pass back what I wanted. And so on with multiple files, sub-directories, etc.
So, I did that, went to write the little bit of code that I needed in the real version (to use the real GetFiles() calls and package the data up), hooked it up to my real code, and it worked.
*But*, when I went back and looked at the code, I found that what I had really done was create two sets of code. There was the real code that called the system routines and shuffled the data into my wrapped classes. And then there was my mock code that let me control what files and directories got returned. But there wasn't any common code that was shared.
So, my conclusion is that I really didn't get anything out of the tests I wrote, because the tests only tested the mocks that I wrote rather than the real code, because the only real code was the code that called the system functions.
In this case, TDD didn't make sense, and I will probably pull those tests out of the system.TDD may make sense the next level up, where I've written a new encapsulation around directory traversal, but it seems like the only code there is hookup code.
So, the result of my experiement was that, in this case, writing the tests was the wrong thing to do.