Great news - this is now available 
We’ve recently released a brand new racking tool on the mobile app. The new tool shares a lot more functionality with the web-based version of Breww, and has support for our app’s built-in barcode scanner and more.
You should be able to rack part-filled containers using the mobile app now, including scanning tracked casks/kegs.
We’re in a period of testing the new mobile app racking tool, so for now, you can use either racking tool. We plan to remove the old tool soon, so if you have any feedback on the new version, please let us know as soon as possible at New Racking Feature Feedback. Thanks!