One of the best ways to get a half decent Nylon string sound is to use something like a schaller Oyster pickup.

http://schaller-electronic.de/hp31356/Oyster-721.htm

You don't have to follow all the manufacturers instructions. I stuck one on the bridge of a nylon string for a gig once and it sounded good (not amazing, but passable) straight into the PA. It does work with regular guitar amps to but you've got to experiment and play with the EQ and try different things like EQ and mounting positions.

EDIT:

Found somewhere else online with them:

http://www.touchstonetonewoods.co.uk/ttwp46.html