yes, a couple of suggestions
first of all don't use the env and vca, this will not work. the stitcher automatically uses the dynamics of your zither.
second i would put the hp first, then the lp (not sure if that makes a difference)
what is the highest note you can play on the zither? is it really 2.96k? i would assume it is quite a bit lower, set the lp to a value slightly below your highest frequency. probably you have to cascade 2 or 3 of those lp filters, since the slope is not steep enough.
let me know if that improves things. i have not yet tested it with a guitar here.
also note, that you may need a gate on the input, since noise from the single coil could trigger the octaver. i don't know if there is a gate in the library though...
the wave input on the stitcher can be feed directly from the input or after the filtering, depends on the sound you are after.