if you look at the current in the circuit there are 4 possible states
1. 0
2. v/r3
3. v/r4
4. v/(r3+r4)paralleled
for the op's idea to work the original re must always come to rest (detent ) where the a b contacts are both o/c .
the pic must also leave the outputs in a high impedence state (ideally with pullups disabled) or the pic will interfere with the re action.
a cro image of the re pins at rest, rotating cw and rotating acw would make things easier