Just searched for this yesterday myself:
http://www.ageod-forum.com/showthread.php?t=4656&highlight=belle+boyd
I can confirm that she will be captured the turn after you put her in command of a unit. Whether she helped with detection, I couldn't really tell. Mostly for flavor I guess but a nice touch. A very nice touch.
Totally off-topic but another thing I just noticed for the first time today: When the CS European bgde arrives in New Orleans it is composed of 5 militia regiments; the bgde symbol shows the unit dressed in what I refer to as the "dress whites" of much of the lower south militia. However, once a majority of the bgde upgrades from militia to regular infantry, the uniform on the bgde symbol changes to the nice gray uniform with red trim! A very nice, subtle touch and just another minor detail that make this game so great and insightful.