Apparently this is one of those things that never seems to be solved. I don’t know why it seems to be technically impossible to create vst-plugs that communicate with other instances of itself and actually initialize nicely when loading a project. Apparently it doesn’t work perfectly with sidekick3, dBProComp or TC compressor/limiter. We’d like an alternative that ‘just works’ without quirks.
My first way of creating the pumping/signal ducking effect when I used Cubase was using a vst effect-version of microgater and by inverting the envelope and triggering the plug by midi (simultaneously with a bass kick) I got the desired effect. As Renoise doesn’t support midi communication to vst-effects and from what I know has no currently plan to be included anytime soon, this method currently doesn’t work in Renoise.
The other method I used was a combination of using Renoise sawtooth-lfo triggering the volume of the “ducking”-channels but as of 1.8, the LFO-curve has been interpolated (or a bug was fixed) which gives the effect that the very start of an sawtooth-lfo isn’t silencing the output and it creates an undesired effect when faking sidechain. In general the LFO is improved, just not the way I ‘misused’ it. 
What am I saying with all this ranting? Well, either a builtin-sidechain effect or MIDI-support for VST-effects would be very very welcome!
Question to developers: Is MIDI-support (note-on/off and so on) for VST-effects hard to develope, as it’s already (naturally), supported for VST-instruments?