Personally I do not know of any medieval accounts containing descriptions of soldiers feigning death. However, the likelihood of such a tactic succeeding would have been exceedingly small within the context of medieval warfare.

First of all, the way in which the battle would have been fought would make it very difficult to feign a death. Medieval combat was not very mobile, the battle lines were closely formed and combat itself was one of direct physical confrontation. A man-at-arms who dropped his sword and layed down on the ground (assuming of course, that his fellow soldiers did not notice his charade) would have been in serious danger of being trampled in melee of the remaining soldiers during the remainder of the battle.

A noted feature of most medieval battles is that the overwhelming majority of the casualties would not fall during the melee between the two opposing armies. Instead, nearly all of the casualties were inflicted after one of the armies would begin to break and rout. During the rout, most of the enemy soldiers would have been run down and killed. In most medieval battles, no quarter was given to common soldiers as this would only be a burden on the victorious army. Only (high ranking) nobles would have been spared; in order to collect a ransom. The killing would generally continue following the battle until nightfall made it impossible.

The following day would see the bagage train and camp followers of the victorious army continue what they would have already have started following the rout the previous day: the killing of the wounded looting of the fallen soldiers. For a soldier faking death, this part of the battles aftermath would present the biggest problem. The Middle Ages, especially the Early and High medieval period were a period of scarcity: everything of value would have been stripped of the fallen soldiers. There was no shame in it and an unavoidable part of a medieval battle: the Bayeux Tapestry depicts this process and shows soldiers piling up the bodies of the dead and stripping them of their valuables. Up until Napoleonic times, a battlefield would be filled by naked corpses; everything else would have been taken by the enemy army and the locals.

Should a soldier succeed in feigning his death during battle, avoid being trampled and the stripping of his body by the camp followers ... he might have still have found himself in hostile territory. Which would make finding shelter, food and the way back to friendly lines (if these existed) very difficult.

All in all, if a battle was lost, generally during the rout soldiers would try to make it back to their camp to reform so that an organised retreat could be organised. If this proved impossible, they would form ad hoc groups of survivors (for protection) and try to make it back to friendly territory. Trying to escape such a battlefield alone, would be close to impossible.