In Ken Follet's fictional book Pillars Of The Earth, a monk who visits the king of England during a civil war in the 11th century is forced to change clothing with the king so that the king may survey the enemy fortifications without risk of attack. While this story obviously never happened, are there any known examples of noblemen in times of war who purposefully disguised themselves so as to avoid being targeted by their enemy?
Not in war per se, but Richard I reportedly was captured in Austria while pretending to be a monk or a lowborn. and caught because of the fine jewelry he was wearing despite other appearances. Indeed, Richard I's supposed penchant for disguising himself as a commoner is likely where the trope of a king disguised among his subjects originated from, at least the folk-tales associated with it.
The Swedish king Gustav I Vasa did this before he became king. He was a Swedish nobleman during the Kalmar-union, which was a personal union between Denmark, Sweden and Norway. The Danish king Kristian II ruled the union and in 1520 he orchestrated the Stockholm Bloodbath in which he executed a lot of Swedish noblemen, including Gustav's father.
Gustav went into hiding and fled in to the region of Dalarna where he tried to gather support for a Swedish uprising. There are a lot of stories about his time in Dalarna. Several of them include him dressing up as a farmer or servant. In one story is he recognized for wearing the farmers outfit on top of his finer cloths, his golden collar became visible while he was working in the farm.
I can tell you shortly about the time he visited an estate outside of my hometown. He had an old friend, Arendt Pehrsson Örnflycht, who owned the estate, so he traveled there, dressed as a farmer in traditional Swedish clothing, hoping for refuge, which he was granted. But what he did not know was that his friend's father in law secretly supported Kristian II. The father in law sent for troops to arrest Gustav, but when the troops came Arendt's wife helped Gustav flee climbing down the second story latrine.
In the end Gustav did convince the people of Dalarna to join him and together with support from Lübeck did they Liberate Sweden and overthrow Kristian II. Gustav I Vasa was crowned the 6th of June 1523.
My sources are in Swedish: Alla tiders historia Maxi, Almgren et al. Svensk historia, Henriksson. And Nationalencyklopedin.
But the english wikipedia page has a lot of the information: http://en.wikipedia.org/wiki/Gustav_I_of_Sweden