You may want to check out the DiMarzio D Activator 7 pickup. Here is the description from the DiMarzio webpage:
"Active humbuckers have a distinctive sound: powerful, clean, open-sounding, with excellent harmonic overtones and very tight lows. This also fits the description of a great 7-string pickup, if the problems associated with active pickups sterile sound and limited headroom could be eliminated. The D Activator 7™ bridge model does just that. It has the same loud yet clean sound as the best active humbuckers, and killer harmonics. The low B string is all about attack, not mud. It responds instantly to hard or light picking. In a word, it rocks. No batteries required."
That being said, I ordered an Evo 7 for my RG8527. Perhaps if I ever have a "metal-only" seven string guitar, I would definitely consider the D Activator since I wouldn't want to deal with active electronics and batteries.
You may also want to go to http://www.sevenstring.org
and check out their discussion forum which has many threads regarding pickups for seven string guitars.
Good luck with your decision!