I'm not sure if it is a common
counter to Waki Gatame, since most times when Waki is used in Judo it's from on the floor from my experience. I have seen some folks try
to get the position I indicated above to get off a counter to Waki though, part of which led to my saying that the setup was semi instinctive (i.e. the turning motion to avoid the arm being locked out fully). The thing is when Waki Gatame is done against the elbow while it faces upward it becomes effective (i.e. painful) quicker than the forward facing version as a good bit more turn is required on the arm for the latter. The attacker has a bit less time to figure out what is happening and start to turn and squirm out of the lock. The opening for a forward roll escape is there, but if one pulls the waki gatame closer to one's centre (tightening the lock on the elbow) the attacker's elbow will snap from his own attempt to do a forward roll. In fact I use the roll escape a bit in Judo grappling, but I only risk it after I know I've loosened my elbow by this turning motion, allowing me to roll with my arm alongside my body instead of at an angle which can make it easy for the other guy to reapply the Waki Gatame.
I agree with you that when Waki is applied well that is usually the end of the match, the same goes for Aikido competitions too I think. It's a bit harder to get off in Aikido comps though.
Just my few cents.