After tinkering around a bit, I think I found a workaround that will allow you to calibrate your MacBook Pro's XDR display using the built in Calibration Assistant…
The Display Calibration Assistant app is still sitting on your MacBook Pro's hard drive, and is located here:
View attachment 1881203
Go to this folder, locate the Display Calibrator app, hold down the option key then right click on the app and select ‘Open’ (this will launch the calibrator in Expert Mode).
Calibrate and save the newly created color profile as you normally would using the Display Calibration Assistant.
Now launch the ColorSync Utility app in the Utilities folder and click on 'Devices' on the tool bar, then click the drop down arrow next to 'Displays' on the left side of that window.
Click on 'Color LCD' (this is the default profile that your MacBook Pro is currently using), and then go to the right side of that window, and click the drop down arrow next to 'Current Profile' and choose 'Other'.
View attachment 1881205
This will bring up the folder window where the default color profile (and now your newly created color profile) is saved. Select your newly created color profile and click 'Open'.
The system will bring up TouchID for authentication, but on my MacBook it just sort of hung there, so I chose ‘Use Password’ instead.
Your new profile should now be selected and will be the main color profile until you create and select a new one using this same process.
This has worked for me and has stayed put after a few restarts, so hopefully it works for you all as well.
So many unnecessary steps when we could just hit two buttons had they not removed (or hid) this option!
Oh Apple... ‘It just works!’