There is no limit to how often you can use the nudge feature. To achieve high scores you need to be using the nudge feature a lot. Nudging can help you to steer the ball away from hazzards, and also guide the ball into targets.
So, now that the Pinball challenge is over, I'd love to hear what your tips were for achieving high scores. It would be especially great to hear from some of the super high scorers.
My own strategy was to shoot for the boat docks section of the game board. This seemed to be the area that awarded the most points. I also relied on the left and right nudge a lot, but didn't make use of the forward nudge.
I never used the forward nudge, but I did the left and right...a lot. I didn't try for any special targets until the 4th board, then I only tried to avoid targets, send it up the left side and down the waterfall for 40,000 points at a time. You can direct the ball down the waterfall with the nudge buttons.
Good luck!