All that is SO much more work than rigging a proper set of cables, and with the servo in the float you'll have the servo wires looking like a scale set of heavy-duty jumper cables hanging out of the plane.
Au contraire Jim, I beg to differ.
By the way, the "balloon" does not have to literally be a balloon. I use latex gloves now, although used balloons in the past. The fingers of the glove are especially easy to tie off. It is super quick to do, much easier than rigging up cables with their attendant fussiness.
The trick is SO easy to do, I usually use it for the electronics installation inside the airplane too, especially for those I fly in salt water.
The 'jumper cable' look might be a problem on very small airplanes, but is not noticiable on anything say, .15 sized, or larger. Tie the wires off to a strut.
Tom