the only thing I can think of is to use a long (5 metre) USB cable and a laptop with the supplied software - that allows you to remotely activate video. I also saw a module somewhere, which allows you to do the same thing wirelessly, but it's not cheap by any means...
A whacky-yet-doable solution would be to have your smartphone hooked up to the computer you're tethered to and use a remote assistance software like TeamViewer to operate the USB tether remotely through your device... lol... it would work!

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BrotherBloat
London RC Project
http://www.rc-forc3.comCanon Kiss X5 (600D/T3i), Canon EF 50mm f/1.4 UMS, Canon EF-S 18-55mm f/3.5-5.6 IS II (kit), Tokina RMC 135mm f/2.8 P/K-conversion, battery grip, CowboyStudios Shoulder Pad