This is what I did first, in that order:
cryptsetup open /dev/mapper/sailfish-home myhome
mount /dev/mapper/myhome /rootfs/home
chroot /rootfs
After that you can check with fdisk -l* which partition your sd card is (most likely /dev/mmcblk1p1) and mount it to any empty, unused folder. E.g. /media/sdcard1.
BTW you might want to add this thread to this wiki: https://forum.sailfishos.org/t/post-festum-sauna-4-6/
And thanks for the hint, I didn’t think to look at the developer release thread to fix my own botched upgrade.
* oops, fdisk -l does not work inside the chroot but blkid does.