You could put new beer in the keg. How many brett cells remain would determine how quickly you would see a change in the beer.
You could degas some of the beer and keep it in a mason jar. My first concern is that there is probably a lot more fermentation brett can do on that beer and the mason jar may end up explosively overcarbonated. Mason jars are not designed to withstand internal pressure (although they will, a little). Second, I'd look at how much yeast you have in suspension. A mason jar's worth of beer is probably short on brett cells. You might wait a long time before you see any activity in a keg, especially if the keg remains cold.
IMO the better process would be to buy another package of brett for future batches. You can propagate brett in a starter and pitch from the starter into your kegs. Keep some of the starter behind in a mason jar for future batches. Just make larger starters than you need and keep the excess. This ensures you can control the volume you pitch and that what you pitch is free of sacc.