My suggestion, if you want these pickups bad enough, you have 2 options off the top of my head.
1) Join a band, become very famous and have a recognized name, convince EMG that your custom pickup will sell
2) Buy the pickup you want a 7-string version of. Buy one of the EMG 81-7's or 707's doesn't really matter. Take apart the 6-string pickup, closely analyze everything about the pickup, how it works, what parts are used, and get an electronic measurement on every variable.
Then, from looking at the 7-string pickup disected, extrapolate the variables on the 6-string model, giving you an idea of what should be added to accomidate the 7th string, and build your own custom EMG using the data collected and the 81-7 housing.
As you can see, neither of these options is an easy task. Personally, I'd pick the first over the 2nd, at least until I get my EE degree. If you know someone with an EE degree, or some serious audio-engineering experience, talk to them about custom designing a pickup.
