Dave "Fuze" Fiuczynski has a number of doublenecks, with the top neck being a short-scale (19") fretless and the bottom being a 7-string. And what he uses for that is a six-string bridge, with the very lowest string fixed. So the whammy works correctly. In my opine (agreed to by multitude-dudes) there really hasn't yet been a seven-string whammy - Floyd, Hipshot, Ibanez, you name it - that has been correctly modded to work as well as a six-string. It has to do with geometry, the size of the springs and their cavity, whatever - Which is why I think Fiuczynski's approach is valid.
And the one thing that would work in that way, to do what you want, is the Stetsbar. It pulls the strings straight back over a tunematic, in direct opposition to the springs, not bent over into a cavity in back. The whole thing lives on the top of the guitar. It's normally mounted on the posts of both the bridge & tailpiece on a standard tunematic-type guitar. You'd have to separate it from the tunematic, and mount it far enough back to allow the fanning to occur at the bridge.
They seem very open to suggestions and interested in working with people - if it was me, I'd be looking at putting it on something like a fanned Agile, affecting only six strings. But I'd contact them right away. Being the internet, you can of course find a million people to say it's ugly, it's wrong, it's not classic, the love child of an electric can-opener and a fishing reel*, etc. BLAR BLAR BLAR...
But if you could get it to WORK as a device to aid in playing MUSIC, hey - if the foo shits, ear wit! :hello2:
http://stetsbar.com/index.html
http://en.wikipedia.org/wiki/David_Fiuczynski
*(that actually... umm... I kinda just made that up! Myself! I'm kinda...like, proud of my li'i chilluns :-\ )