Lynx

A carefully selected resistor and capacitor would allow you to switch LED's on and off directly from the signal line, at least via a transistor. You'd have to sit down and work out the math or just tweak it, but a far left servo signal pulse is on for 1 ms 50 times a second, and a full right servo signal is on for 2ms 50 times a second. When that's filtered through a resistor and capacitor it translates to a steady state voltage (roughly) The key is getting the resistor right so that the voltage drops bellow the LED's threshold voltage at the far left position. Using a multi-turn low ohm potentiometer should give you pretty fine control. The capacitor would have to be matched to the transitor you used to switch the LED's as some use more current so would require higher capacitor to cause it to switch. A logic level mosfet would be ideal as you could use a minimally sized capacitor for it. The benefit of doing it this way is if you bias your resistor and capacitor just right you'll have a decent level of brightness control. You could build the whole thing on perf board, or wire it up dead bug style after you test it on a breadboard. Really shouldn't be too hard.