May not be a blocker for mus_ash if ash and exo are in the same process. We can discuss this more if there are still benefits to better isolation.

I think that's necessary in short/mid term and even may be better because exo also handles notification stuff, unless it has to be separated.

exo can use the ash container ids even in mus+ash. See examples in chrome (e.g. [1]).

[1] https://cs.chromium.org/chromium/src/chrome/browser/chromeos/bluetooth/bluetooth_pairing_dialog.cc?l=72

(i.e. even if ash and exo are running in different processes)

Ash and exo are staying in the same process, so this is fine.