I was thinking 10% Munich is appropriate. Too much in a light beer will darken the color. Maybe you could get away with 15-20%, I have not run the numbers through software. Depends on how dark you want. If color doesn't really matter, then yeah, use 20-30% with great effect.
Depends on what Munich. The color of some of the Light or "Munich I" grains are as low as 6L. 20% of that stuff in a beer made with 2-row or Pale Ale malt is not going to have it's colored affected to the point of detriment.
Also, 10-15% will give you a slight graininess that is pleasant but not immediately identifiable as Munich malt. I think it's really nice in a beer like this.