The registration number and selcal number and fully implemented in a texture.
With software like Paint.net and Co. you can change it yourself (I did that for myself).
Unfortunately, it is not known whether the numbers from the MSFS setting can ever be adopted or whether the ingenious developers of FlybyWire can manage them.
But would be desirable.
PS. The texture is “A320NEO_COCKPIT_DECALSALPHA_ALBD.PNG.DDS” and is located in the folder “A32NX \ SimObjects \ AirPlanes \ Asobo_A320_NEO \ TEXTURE” from the community folder.