Since apple with BigSur beta 5 (I guess on purpose) reduced the BaseSystem ramdisk size (available space), I adjusted the "BaseSystem legacy usb fix" that still produces a working installer for non-APFS or legacy USB Mac:
BigSur BaseSystem legacy usb fix.zip
(stock beta5 BaseSystem.dmg size 745 MB , after legacy usb fix is 700 MB , on beta 4 was 840 MB )
so "patched prelinkedkernel" works for the beta 5 installer and allows to install BigSur beta 5, but not yet to boot BigSur after install or upgrade, you might need USBOpenCoreAPFSloader4 and CMD+S with "exit" to boot legacy USB Mac.
While for any recent USB Mac, there are no issues in using the BootKernelExtensions.kc , to apply kext patches use the micropatcher from its USB BigSur recovery environment.
Another weird working way to disable "snapshot booting" that means a working "sudo mount -uw /" is to skip the stage3 installer, I used it also for this beta 5 and it worked too.