To the post above, Im pretty sure Nadal uses extreme semi western with his straight arm forehand. I think the straight arm forehand can work with more extreme grips its just that the transition between pronation -> supination will be more violent, could possible cause discomfort. Although, unless you're a top spin junkie, theres no point trying to utilise the straight arms inherent ability to create more topspin if your balls have no pace
So yeah I think semi-western to eastern would be a good grip range.
BevelDevil is spot on.
If you really want to learn it, be prepared to train and practice for a long time. Best way to tackle it is with small steps.
1. If you don't pronate on the takeback of your forehand with a double bend forehand, learn how to do that and do it fairly consistently.
2. Once step 1 is done, the next step is to practice your footwork so that your contact point is slightly more out in front than usual. As you get more comfortable with it, increment the contact point distance further. This will help with getting used to the positioning of the forehand. This is important because you cant hit a straight arm forehand when the ball is too close.
3. Lastly, learn to relax your arm (not too much) once the backswing has started throughout the stroke. This will cause natural arm extension and wrist being laid back. The wrist is like a hinge, its not consciously controlled.
Theres a thread on Fed's backswing around here by tricky, you should have a look at.