Just for the record...

I did a standard system install of wheezy on encrypted lvm, then added
xorg and xfce4 (full metapackages) and not much else.

Changed sources to devuan jessie and added the avoid-systemd pin, ran
'apt-get update' and added the devuan keyring, then ran 'apt-get
dist-upgrade' and booted into the new kernel, and all was well.

Easy.

fsr