How about the following suggestion:
How about a "hanging leg lock" type of sub attempt. If the opponent is in a "sitting guard". Grab the opponents left shin/lower leg (or whichever leg is protruding more), with your right arm. Then pull it hard up into the air and latch onto it by curling your arm. Now with your left foot you step on his inner thigh of the opposite leg and hold it down by putting your weight on it. Now you're kind of stretching him out a bit. From here you could go for a standing type of ankle lock, or achilles hold. Or you could quickly take him into a heel hook position on the mat...either go for a standard heel hook kind of position, or you could step over the leg that is pinned to the ground and go for the heel hook after you pass the opposite leg.
Or, if he has one foot down on the ground and is in the sitting guard, you could place one hand on the top of the foot, and put your weight on it...say, your right hand on his left foot for example. Now, in one quick motion, you figure four the ankle/foot in one quick motion and then dive your body over his left leg, all the way to the outside (without hooking the other leg) and roll into an inside toe hold. I've seen this one done by high level people in tournaments a couple of times. Once that figure four is locked it seems pretty secure and you really don't have to bother with hooking the opposite leg. It's faster, pretty secure, and a quick sub.
Here's an original one that i created for a guard pass. Since i'm (very) good with arm drags, i've adapted them to the leg. Now if the guy is in the sitting guard with the lead leg bent a little and forward, what you can do is grab high on the calf, by the knee in one very quick powerful motion. Next you quickly pull that leg towards the other leg at about a 45 degree angle. This actually pivots him on his butt as the momentum spins him around. One reason this works is that a natural reaction to the "leg-drag" is to pull your leg back into yourself. This makes the leg stiff enough to be successfully draged........Now, after you have pulled/dragged the leg toward you and the opposite leg at a 45 angle, you actually can go for the side mount right away, or you can kind of roll into him. So if you are dragging with your right arm, you will roll onto him with your left shoulder leading. You roll all the way until you read your stomach. Now you'll be in a high side mount, or in the north/south position. With this last method he has a harder time in getting his inside leg back in to re-gain guard.
Anybody else have other unique ideas???