To clarify, as I didn't see it in the posts so far:
If you want to retrofit rear speakers in the facelift model, you have to splice them into the audio outputs of the front doors, which is not ideal.
The engineering menu exists on the facelift models as well (2 different menus for Ismart lite and Ismart) but it does not offer any option to change configs anymore.
I am one of the first (maybe the first) private shops in Germany with full OEM workshop manual and diagnostic access to MG including all module updates and configuration.
I can confirm, that even through the OEM tool, there is no way of activating this. Modules are configured automatically, the VIN is sent to SAIC, they return the equipment level details/matching config and the tool writes it into the module.
It's also not an option to use a module from a better equipped car, as the VIN is one time written into the module and locks it out to be used in other cars.
The only long term solution will be third party software to code.
Best
Sascha