In a small saucepan, simmer the rhubarb with 1/4 - 1/2 cup water for 5 minutes, or until very soft. Remove from heat and set aside to cool. (I've done it warm too, and it's just fine.)
Put the rhubarb into a blender with the honey, vinegar and mustard. Pulse until smooth. With the motor running, slowly pour in the oil.