I have never taken the back from de la riva. I understand completely what you are doing and find it amusing that I haven't done that before. I have done a similar pass from standing but never on the ground. Glad I read that. I get de la riva all the time. At the state championships this year I won my last match with 2 points from a de la riva sweep. I couldn't pass the guy once I swept him but got the 2 for the win.
I also have been working on taking the back from the guard. Most of my techniques are derivatives of Roger Gracie (I am not quite as tall but not far off in total size) moves. I had a private and worked on them for an hour.
I can now take the back from closed guard using a simple grip. It turns out that the grip is the key part. You loosen the gi on the right side, sit up into your opponent and reach around their back with your left hand and grab under their right armpit. Ideally with your right hand you pass their gi to your left hand and you can grab it tight right under their armpit. From there just lay back to your right side open guard and use your right leg to push their right knee away and pull them back into you with your left arm. Voila great back control.
I am enjoying the blog. You ought to do a shameless plug in the fighting section or at least here in your log.
What we think, or what we know, or what we believe, is in the end, of little consequence. The only thing of consequence is what we do. -John Ruskin