No doubt, this kind of attack is very common. Not very smart to leave your arm out after a punch... I practice doing irimi at the same time uke launches. Hard to explain but I'll try;
If uke punches with the right arm, I pivot while stepping slightly offline to my left. While entering, attack uke's center line (towards uke's face) across their extended elbow with your left arm. Use your right arm to parry uke's strike. This happens very quickly as you are entering at the same time uke is attacking. Usually this is enough incentive for uke to keep the striking arm extended because if uke draws it back, uke eats a fist. (isn't it amazing what the brain will sacrifice to save the head from getting hit?)
I usually get three different results from doing this,
1. Uke draws the arm back anyway and gets hit in the face. Then put's it back up to protect the head from further blows. (your choice of technique here)
2. Uke changes purpose of striking arm from attack to defense
( this is the usual response. again, your choice of technique from here)
3. Uke trys blocking your atemi with opposite (left) hand. (classic setup for ikkyo)
This of course is what I practice in the dojo. In the "street", I like to de-fang the snake first when it comes to boxers or "strikers". (I've used this on both types with much success) That is, close your hand, using the middle knuckles of your fingers (like knocking on a door), atemi with a whipping motion and strike the back of the persons hand as they jab or better if you can catch them posing for an opening with their guard up. Boxers love to keep their hands up thereby making their hands available targets. If you strike hard, they will naturally pull it back which creates an opening for you to enter. Usually you end up breaking bones in their hand which tends to discourage them from throwing it out again. IF YOU PRACTICE THIS IN THE DOJO PLEASE BE VERY CAREFUL as the bones on the back of the hand are very close to the surface and break real easy. It doesn't take much of a strike to really do a lot of damage here. If you want to practice this little move, try slapping the back of uke's hand with an open hand. The motion and timing is the same.........
Train hard, Play hard, Live easy.
Dan P - Mongo