* What I understand from reading i2c-demux-pinctrl.c, a slave device
  will only ever be observable from one master at a time, when you
  switch over, all children get removed on one master and added to
  the other one, to be probed again by their respective drivers.

Yes. The very first versions of the demuxer tried to do it in a
hot-swapping like fashion but then I switched over because of...

it definitely opens up questions in what happens to e.g. the sysfs
representation, lock order, and power management that I'd rather
not have to think about.

... this! There are dragons, I can tell you :)

* The other thing we definitely have to support for i3c is to deal with
   handing over control of the bus between the i3c master owned
   by Linux, and other masters that are /not/ owned by the same
   Linux instance. This is the part that the spec discusses in much
   detail, with the intention of temporarily giving up control of the
   bus to let another master do its thing on a shared slave without
   user interaction.

I can't comment about this one.