The technique you have 'stumbled' on comes from the 3rd section of the Hiden Mokuroku of the mainline Daito ryu school under the late Takeda Tokimune sensei.
It is called waki garama and is initally done from a single collar grab from the rear. I'm not going to look it up but I think it's a variation of shuto zume which starts from a frontal attack. Both involve wrapping the arm and then pressing the locked elbow down.
These waza can be found on the Seishikan video / dvd's thru www.Budovideos.com
Just remember that Takeda Tokimune reorganized the Hiden Mokuroku so they may fit slightly different into the curriculum to older Daito ryu. Ask someone who studies DR for more info, I'm just a curious Aikido guy.
As you view more of the techniques of DR in video, you realize 1) they pretty much threw everything in the technique bag and 2) there isn't anything new (if you go looking far enough) under the sun.
When I started breaking out of the 'mold' of the limited Yoshinkan curriculum, I started coming up with 'new' stuff. Sure enough, I soon discovered this or that in other systems of Aikido or Daito ryu tapes.
Now, it's great to experiment, and there are always new applications of old waza. I always say that Aikido holds alot of the jigsaw pieces of DR but you have to go looking for the corners and the center pieces.