There's two options... one's Duncan's
Custom shop, who will make you a stacked single coil to match humbucker volumes. I have one. If you want active pickups you'll still end up sacrificing. EMG had it on the tab;e to make a 7-string SA and unveil it at NAMM in January of this year, but they opted not to.
There is a guy on this board named Slanman. He cam into my shop and I did the mod on his Universe. I installed an EMG 707, and a EMG 35TW. His original middle pickup is still there and works with the EMG's even though it's passive. I've done this mod to several guitars...
There are those who will tell it doesn't work. Find Slanman and ask him...it can be done...lol
P.S. Slanman, if you're around, gimme a shout man! Haven't heard from ya' since the vid company switch.