1) By default configfs is not mounted, so I need to do it from some snap because this is the only customization point for Ubuntu Core.
a) If you try to /bin/mount from shell script than this gives immediately DENIED with suggestion to "adjust snap to ship 'mount'". It seems like a dummy message which will never help, isn't it?
b) /etc/fstab is readonly
c) trying to call mount() syscall programmatically will give a SECCOMP deny, I suppose?
2) And then configure the system with configfs which requires write access to it.