ok so i've read about people's problems with transitioning from bent-arm to straight-arm frog stands and have the same problem. so...i finally made some progress on it and wanted to share what i feel has been helpful for me.
basically this is what you can try, it works well for me.
ok get down on the floor like you're going to do a frog stand. (you should really stretch out your wrists first as this will be quite stressful on them). try to position you knees as high as you can above your elbows. then, with your elbow-pits (the inside of the elbow joint) facing outward, lean forward, while still keeping you toes on the floor. while you do this you should really try to take the weight into your wrists and elbows. what i mean is that you can use the weight of your legs to actually *make* your elbows stay straight. it almost like your relaxing that place while contracting like crazy in your abs. if you lean forward enough you should feel a lot of work going on even without lifting your toes off the ground. you can also try lifting one foot off the ground at a time. that's the stage that i am at.
after writing it down it seems a bit complicated, so if there are people who want to see it demonstrated, let me know.
good luck in training...