Very cool. Always enjoy seeing virgin forge world bits.
Another idea popped into my head while reviewing this. I remember seeing a custom commission of Horus a while back. It may have been on this forum by the Den of Imagination studio (Brovatar)....maybe.
Anyway it may have been a Luna Wolves era Horus with mace and wolf pelt on his back. The pelt specifically was made from green stuff to fantastic effect. If you're investing some serious time/resources into your SW, I would recommend picking your warlord HQ and converting it + green stuff pelts. Perhaps as one of the final touches for the army, but would be awesome imo.
I will try to find and post log if I can.
Update: Found it :D Insane work from Brovatar as always