You can detect signal two blocks above using pistons, however you have to update it regularly (without powering it) to check for change in signal. The other option could be using xychoridite lights and light sensor, but lighting it up doesn't seem to update light sensor so there is the same problem as with pistons, RP2 lights and normal redstone lamp work, but can't form tank.
Edit: Redstone block won't work, because it powers all the time, the only way to control it is to move it using pistons. The easiest solution would be to just move lever to bottom layer of the tank and just power redstone below it or place lever on the tank side and get signal this way.
Edit2:
This works:
http://imgur.com/a/mybfw , timer is just to update piston when input changes. However I'd just do something like that:
http://i.imgur.com/qDciNCR.png