Ikkyo on the ground- nage flat on back, uke in full mount. Uke rears back for the punch, nage's hands are up around his chest area, nage's feet are flat on the floor with knees up. Uke delivers punch (energy going downward with likely more than a slight bit of offset from good posture) at nage's face, nage deflects/guides uke's hand away from face so that uke's arm crosses his body. In a continuation of the deflection across uke's centerline nage bridges in the same direction uke was deflected in with one leg (Uke's right hand would mean Nage deflects with right hand to right and bridges on left leg) and rolls across his other leg onto his stomach and brings the other hand up to cut at uke's elbow ikkyo style. Essentially nage uses uke's momentum from the downward stike to create the kuzushi then rolls in the direction uke was just predisposed to go. This roll will leave uke extended on his stomach at around a 40 degree angle of of nage's centerline with his arm extended in a full blown ikkyo. Nage will shrimp up using legs to bring uke's arm closer to his own center then apply a gooseneck/corkscrew (palm facing uke with the wrist twisted so that the elbow is exposed even further and the radius and ulna come into opposition against eachother) and apply a cut with his left hand while raising uke's corkscrewed arm slightly to provide a better angle for the cut.
Situational. I suppose it could work. Maybe on an inexperienced guy. I am able many times when mounted to work sankyo on an inexperience guy and shrimp out. It only works a few times however and in NHB the risk of getting hit is too great to work it.
I'd say your scenario would work on a guy who punches off balance possible. An experience guy won't do this. I'd agree with Michael on the Arm Bar deal with your arms extended up.
Most guys will ride up into your arm pits prior to punching you...negating your ability to roll or do any technique at all. also, a experience guy will keep good posture and control when he punches so you aren't going to grab his center...he has yours!
About the only thing you can really do it start brigding. Maybe you can gain control of an arm...trap and roll him to the guard...that is usually what happens.
As far as ikkyo...one of the best examples in BJJ is the omaplata...(just finished doing three submissions about an hour ago!)
I was also working with a greco roman wrestler today. He showed me a russian...he said it is the basic starting point for all take downs in G-R. What is interesting about it...is that it is Ikkyo!