You can double-click on the ground behind the enemy line and your grouped cav will, I suppose, just run straight forward without bunching up, but in that case they wouldn't charge; they'd just run into the enemy and start fighting without getting the charge bonus.

It's a shame this doesn't seem to work, because I had a ton of fun playing with massed cav charges in the BI-based mod Fourth Age: Total War. You could just line up all your Rohirrim, group them, and send them flying at the enemy. That's the kind of thing I'd like to be able to do with my Templars in the Crusades campaign, but it seems I'll have to just pause and micromanage each individual unit's charge. Unless someone knows something I don't!