That's true. There really isn't a good way to communicate with your userbase. You're basically limited to updating the app description and hoping your existing users bother to click the "More" link to read it (assuming you didn't put the update at the top, because that's prime real estate for attracting new customers).
It really astounds me hearing about how long new apps and updates can get stuck in limbo, with no feedback from Apple or path to action. So far I've managed to avoid any nightmare scenarios, but every new anecdote I read makes me increasingly anxious that one day, that time will come.