The drawback is that if you try to make a vibrato with such a pitch wheel, the vibrato will become "deformed" due to the non-linear nature of the wheel.
Incidentally this is actually the idea behind the Nordlead's odd "wood stick" pitch controller, to eliminate this dead spot.
The first idea I had was to just add a control which then feeds a differentiated voltage into the pitch wheel input of the synth. The easiest way to accomplish this is simply drill a hole somewhere, mount a potentiometer and feed the signal from its center tap thru a capacitor and then to the center tap of the "real" pitch wheel. That's basically all there is to it.
On the DX7 I had to adapt the circuit a little, because one additional way that the pitch is centered on zero in the DX7 is by feeding a voltage to the center of the potentiometer (the other way is with springs in the pitch wheel assembly that returns the wheel automatically to center position.) This voltage is stabilized ( = from a low-impedance source ) and the op-amp in the stabilizer, like it should, completely absorbs any output from the capacitor. The quick fix is to put a variable resistor in series with this voltage. Thus, when the pitch wheel is in the center position, current from the capacitor can flow thru this resistor and affect the pitch voltage.
How it looks
Where to get to the +15/-15 rails easily
Experimental force translation device (note high-tech spring mechanism <g> )