you might be inviting more problems than you need to here.
first, you'll need to get the signal TO the neck or headstock. (of course it CAN be done, but it does add complexity to the construction of the neck.)
second, the relationship between the origin of the mechanical energy and the string will change, depending on where the string is fretted, or if it is open. that will how well the system is coupled.
i THINK you will be better off, having your mechanical system return the energy near the bridge end of the string.
cheers,
unk
ACOUSTIC SUSTAINER ADVANTAGES: The acoustic type of Sustainiac Sustainer (MODEL C) produces the most natural feedback sustain. This means it is most like getting feedback from a big, loud amplifier. It puts sound energy into the guitar headstock, which then gets transferred to the strings. Feedback is very intense and instantaneous, unless you purposefully have the intensity controls turned down. Most played notes will change into a harmonic after a short time, like you get with natural amp feedback. However, the MODEL C sustain is not all fundamentals in one mode and all harmonics in another mode, like the magnetic sustainer (STEALTH & STEALTH PLUS) does. Furthermore, unlike the magnetic sustainer, all of your pickups will function normally when you use the of Sustainiac MODEL C
The acoustic transducer design vibrates the headstock of your guitar in response to the amplified played notes. You can feel it when playing. The intense neck vibration makes the strings vibrate very intensely. This makes the amplified signal stronger, and so on. Feedback builds up quickly.
You can but more energy back into the guitar thru the headstock end simply it's easier to move ( vibrate