How can I mount the root Data volume as writable on macOS 10.15 Beta 1 when booted into Single User Mode?

I need to reset my admin password (https://apple.stackexchange.com/a/61932/17533). Can't boot into Recovery Volume, so I need to boot into Single User Mode and modify the Data volume.

When booting into Single User Mode, the system shows the following help:

To mount the root device as read-write:
    $ /bin/mount -X /

However, that leads to an error message:

mount: illegal option -- X

I also tried the old mount -uw -, but that leads to some firmlink related messages, and afterwards I still have no writeable Data area.

mount shows that the Data volume is already mounted at /System/Volumes/Data as read-only. Trying to unmount it with umount leads to an "Operation not permitted" error.

mount -uw /dev/disk1s1 (which is the Data partition as shown by mount) leads to the error "Found 2 volumes in group ... Failed to stat file //AppleInternal, error No such file or directory".

I remounted the root first with mount -uw / followed by mount -uw /System/Volumes/Data to remount the data overlay. Be careful not to add a trailing / because it'll fail with "unknown special file or file system" /System/Volumes/Data/

Both issued a bunch of error messages, but everything was writable after that (as verified by the output of mount and also by touching some files in / and ~.