Until yesterday I was using a complicated setup with two crossovers and 4 broadband compressors in a free-linking VST host. Then I found this: wavearts multidynamics
It supports up 6 six bands for multiband compression and is a simple stereo in, stereo out VST plugin that can be used in a simple VST chainer or even a player that only loads one VST at a time. I split the sibilance range from 2500Hz to 16000Hz into 4 bands and leave the top and bottom bands untouched.
Here is the preset I made:
When the VST plugin is loaded with this preset, just adjust the threshold for all the bands (the scroll wheel second from left at the bottom of the Multidynamics interface) until the compression kicks in at sibilant points and not elsewhere.
If one is looking for something similar there is the spitfish plugin