Just found out there's an official Windows desktop icon pack for Apogee/3D Realms' games:
http://www.3drealms.com/news/2001/02..._3d_realm.html
Here you can download a zip with the original file date on the archive:
APOGICON.ZIP
Some search came up with other packs at cd.textfiles.com:
APOGICO.ZIP
APOGICON.ZIP
These two don't seem to be official, although I'm not sure about it.