Hi Steve, perhaps there is a better way but I believe you can acheive the above by setting up and saving your build.yml once for each configuration. For each time go to the RhoMobile workspace for the project and copy over the build.yml and save it. For instance from within your project's root directory - Drive://workspace/myApp/build.yml - create a folder each platform and save buld.yml in each of the folder. For example:
Then you can retreive the appropriate build.yml file and restore it in the root of /myApp before building the app for the desired platform.