You can only get 'classic' brett flavor (ie Orval) from secondary fermentation under pressure, so a keg is a great way to go, even if you plan on bottling. You can expect 1 vol of CO2 from every .002 drop in gravity. 1.000 is a reasonable assumption for FG, but it could be lower or higher depending on grist, mash temp, etc. Good thing about trying this out in a keg is that you can always vent pressure if need be. I don't keg, but bottle with brett some. A friend just did a white IPA with brett in a keg and he wished he'd have bottled it after I stopped by. Dangerous having something that good on tap!
The only thing worse than dumping beer is serving beer you should have dumped.