.There's no pattern that I'm aware of, but you can do a search for WEP G-8 patterns and see if anything comes up. As you may or may not know, the jackets used on SG-1 were based on a flight jacket called the WEP G-8 from around the Vietnam war era. The jackets used for the pilot, "Children of the Gods" was an unmodified WEP in its original satin like material.
I'd also contact Gweneal on the forums here, he's a very talented costume maker based in France and has made a number of his own Stargate costumes. i don't know if he's made his own WEP but it can't to hurt to ask if he has and, if so, if he's willing to share his pattern with you.