JP, I was thinking about you as I did this pass at lunch today. Doesn't that give you warm fuzzies down inside? ;)
As Andrew said, as you smash down on his legs and pass around that bottom leg is the one you have to worry about.
As far as your top arm being kimura'ed when you reach for his head, good point. If you are lazy with that arm, you will get caught a lot. Therefore, make sure you DIVE that arm behind his head when you go to lock down his upper body. If you move it up towards his head casually, he’s got lots of time to lock up your arm. Take your time, execute your pass with good pressure and control – then dive your arm up to control his upper body when you’re ready.
If he does catch you in the kimura, you have several options:
Back up and kill it – turn your elbow down towards the floor and bring your wrist up towards your sternum. Use your bodyweight to lift your arm up and strip his hands off. This works well early on, but you have to stop your pass to execute it.
Drive forward and kill it – if your left arm is caught, drive your weight forward, straighten your arm a bit, and insert your knee into his figure 4 frame. Extend your hip and break his frame, then continue forward and complete your pass. Sometimes they will try to shrimp away from you to restablish the lock - you have to "chase" his frame with your knee and pin it down if that happens.
Step over his head, spin and take the straight arm bar. I am not an expert at this move, and I generally only try it when I don’t feel I’m in true danger from the kimura. You look really cool when you pull it off, though. Matt Hughes just did it vs. George St. Pierre in the last UFC, and he definitely looked like a pimp.