Hi,
yes, maas 3.1 does provide the ability to define a custom layout for a machine.
This is done by generating a JSON file during commissioning with the desired layout.
Documentation on the format and how to provide the config can be found here: Custom storage layout configuration examples.
We’re workingon finalizing public documentation for the 3.1 release (hence that page was not published yet), so additional/more detailed documentation wil be available soon.