I've done this twice and I've had great results with just using the dregs without a starter. Considering that brett is a slower fermenter, you may get mostly growth of the primary sacc yeast in Orval anyway if you did make one.
As long as your beer is well attenuated after the primary (and my Orval clone definitely was - somewhere around 1.007 I think), you shouldn't have to wait very long to bottle. Orval secondaries with Brett for 3 weeks before bottling, and I did the same. After a year, it probably dropped about 2 - 3 points, so it's better to use heavy Belgian bottles or champagne bottles. I haven't had experience using this in beers with lots of residual sugar after the primary, so I'd monitor it closely before bottle to see if the gravity is still dropping to avoid explosions.
This is a pretty fast-acting brett. Depending on your secondary temperature, you can have some serious barnyard funk after only a month or two. I tried keeping it around 60 and also 75, and I actually preferred the more aggressive flavors from keeping it warmer during secondary, even though Orval uses the cooler temp.