I've just found out that Automatic System Upgrade can now read an UCI OWUT config parameter that tells it to expand the filesystem to the given size:
$ uci set attendedsysupgrade.owut=owut
$ uci set attendedsysupgrade.owut.rootfs_size=1024
$ uci commit
Problem is that the parameter has a max. of 1024 (1 GiB). This should be increased. I have reported it to the ASU team here.