I was delighted that the latest update of WaveLab enabled me to add plugins with extra unused outputs into the output effects slots of a 4-channel (in my case) montage. However, I have not been able to get around to testing it until now.
Positive: Yes, I can install such plugins.
Negative: They do not work.
Quite simply, the ones I need to use appear not to be inserted in the audio chain at all, though I hear a click when they are enabled or disabled - the plugins I have with built-in metering show no activity on input or output, yet the audio at the montage output continues unaffected. This is not only an issue with plugins that have extra outputs I’m not using - it’s the same for 4-in/4-out plugins.
Steinberg’s own plugins that handle multiple channels do work - Gain and Peak Master for instance. But Restore Rig reduces the output to just two channels in a 4-channel montage (which I guess is no surprise).