Re: nikyo (ura) from shomen uchi
It's a tricky one. I've been looking at it a lot over the last few months, as I have to grade on it on Saturday ;-g
Saotome Sensei's book "The Principles of Aikido" includes a good photo sequence for this technique, which he prefaces with the note that "nage must start by trying sincerely for shomen uchi ikkyo"... and then illustrates how the transition from ikkyo to nikyo can arise naturally out of uke's attempt to rise up in the course of having ikkyo (ura) applied. Saotome uses this as an example of how uke and nage work in harmony in the execution of a technique.
Another good resource for shomen uchi nikyo (ura) in my opinion is Yamada Sensei's "The Power and the Basics", where he goes through it in a way which shows the hand positions and the corresponding steps nice and clearly.
As a prevous poster noted, you'd be lucky to get away with grabbing uke's 'striking' wrist directly (i.e. with the gyaku hand). However, Yamada shows clearly how an initial ikkyo-style interception is smoothly converted into a wrist-grip in preparation for the application of nikyo.
That is: from a two-handed ikkyo (ura) style interception, you step to uke's ura side and turn so that you keep facing his striking hand; as you turn, your gyaku hand slides down from uke's upper arm to his wrist (incidentally guarding against an elbow-strike on the way). You are then in a position to sink, drawing uke's striking hand down and around in preparation to apply nikyo.
I hope this description makes sense... also I hope that I can actually do it and not just describe it ;^)