I assume this is so as to use the P:5 gen 1 within an S2 system?

I think you could….. but I don’t think the P:5 would then sync with the speakers in the S2 system, because of the line-in delay.  And if you can’t group it, you might as well leave it on S1.

It may be my thinking is wrong on this so I’d be interested in what others think.

Yep, the S1 Play:5 would be subject to the usual Line-In delay, so ~75ms behind the S2 system.

I guess one possibility would be to split the ZP90’s output, feeding back into its own Line-In as well as the Play:5’s. Then other speakers in the S2 system playing that Line-In would be equally delayed. Not perfect sync with the Play:5, and horribly messy as it would go through an analog stage to get back into the S2 system. Never tried it, for obvious reasons. If the equipment is to hand it might at least be worth an experiment.