Download in iTunes... but only to iPhone?

I love that I can download an app in iTunes and have it pushed to my iPhone. But, I keep my iTunes library on an external drive, and usually I'm on the go and don't have it hooked up. Is there a way to click download in iTunes, have the app download on my iPhone, but not download to my MacBook? That would be rather handy, so I don't have to mess with files on the drive later.