I remember legacy versions of Nuendo (pre-v6) which offered the possibility to phase-reverse individual channels in the Control Room’s Downmix presets. This was especially handy to create a “M minus S”-mono downmix for a quick check of the signal’s stereo-only components. (The monitor-sections of most large-scale studio consoles offer a phase-flip button right next to the mono-switch, therefore.)
As I see it, we now have to use an extra insert effect to achieve the same functionality. If there’s no other hidden trick I would love to get this trivial little feature back as part of an updated MixConvert-plug-in ASAP.