Yeah the Pedal IO might not make sense upon consideration. I was just thinking of other things that use the 3-pin connectors. There might be a fun way to use it with XY as well.
Inputs 1&2 on Mixup are mono so they’re sent evenly to the LR out. My thinking is that if you chained the output from Mixup to the theorized MS EncDec 1U, it would sum the Mixup signal with anything at the LR inputs (or it could be a normalling setup where the LR inputs cancel the Chain In). Then you’d be able to send the MS out for whatever processing you wanted, such as lowpass filtering the mid, highpass filtering the sides, or adding reverb delay and/or chorus to the sides and return it to the MS input for conversion back to LR.
With a Chain output, it could be connected to another Mixup or a Stereo Mixer 1U leaving the LR output free to patch somewhere else to create another potential send return path. Seems like the Chain Out would be maybe be more useful than the Chain In.
The -6db switches or jumpers are a good idea too.