Thanks Macker. You get it exactly.
"I'm not sure what your idea includes when you say 'and copy along with it its native Mixer channels.' "
When the SY slot is pasted into a SYzd dimension tree, the hope was that the SY's output mix channels would be added to the SYzd Mixer tab. So if, in its native SY habitat, the SY patch slot outputted to the common Main / Main-C / Mid / Close channels, those channels would be also pasted into the target Mixer tab.
I'm not complaining. VSL has bigger fish to fry than this, I know.
"...we can't directly copy/paste the Output Config matrix settings."
Yes. In the porting over I described, each Patch Slot had to have its Output Config separately configured. It's not like the Setting page, for example, where setting can be copied and pasted to different instances. I think the ability to cut and paste Output Configs might be a reasonable feature request -- sort of like the ability to drag SY mixer channel's for re-ordering.
"... setting up the extra channel(s) in the mixer has to be done only once...."
Right. It was the only silver lining in the process.
My reason to do this was something of an anomaly. For a two-staff trombone display (three trombones and a tuba), I sought SYzd Dimension trombone three and SY Tuba to sound in a stack for a2 performance on the second staff. Alas, there is no SYzd Tuba, so I resourced the SY Tuba. Hence the merging of SYzd and SY in a single instance.