It's not currently possible, the default commands calls through to a sync function which uses .wait() so there's a new PR (Async Entry and Custom Executor by 0xTim · Pull Request #131 · vapor/template · GitHub) to solve that. I have this working in a sample project so you can use that branch if you want to try it out. (There's a breaking test which I need to fix before me can merge).
The reason why this extra PR is required is because the compiler swallows transitive noasync messages - I've put a pitch up to resolve that here