Hey there.
Cubase user for decades here, but first post I believe 
I’m using a lot of outboard.
An option for MS processing with useful stuff like voxengos MSED offers would be very welcome on my end.
I’ve got a lot more ideas for external processing which I will create individual posts for.
Let me know what you think.
Best
Ilan
EDIT:
And what if you make it happen like this:
Ch 1 - stereo original mix - not sending to master out
Group 1- Stereo - - Receiving from ch1 - MSED inserted and mute SIDE (output to Group 3) - Insert external effect
Group 2 -Stereo - - Receiving from ch1 M MSED inserted and Mute MID (output to Group 3) - Insert external effect
Group 3- Stereo- - Receives from grp2 and grp3 ( Its your Decoder channel) Output to MASTER
The only problem here is if you use a different outboard for MID and SIDE, that will change the roundtrip latency and mess with phases! That’s why i don’t use outboard to make MS processing. I prefer plugins on the session.