You're not going to get a Flanders Red just pitching Brett. FRs have a pretty sour combo of lactic and acetic acids, and you'll get very little of that from Brett. You'd need to add some bacteria as well.
Edit: a guy in my brew club did the same thing. He brewed a Flanders Red recipe and pitched just Brett B or L in secondary and let it sit for a year. It resulted in an amber/red colored ale with a huge horse blanket/horsey flavor (no sourness though). Nothing wrong with that if that's what you are going for, it just won't be a Flanders. Maybe you already know that, but a lot of people don't.