There are special rules for allies in the wonderful and glorious General's Compendium. I believe they're in the multiplayer battles section.
Limiting the Screaming Bell might seem fair. But I doubt that the evil side will like that idea. Making the entire enemy army take a panic check
when you're playing a 10000 point battle hardly seems fair
My favourite is the High Commander rule. In addition to everyone's generals, one is made high commander. All generals allow their own army to use his their LD...but any army can use the High Commander's LD within 18". The high commander on each side is also allowed extra magic item points, we say about 150 to equip since he'll be the target for a lot of things. His death is worth 200 VP (High Commander) + 100 VP (General of his own army) = 300 VP.