I was reading through my Lonely Planet, "Paris Encounter," book and found this tour:
Une Nuit en Metro (A Night in the Metro)

It had this website listed:
http://ademas.assoc.free.fr/

I tried using an online web page translator, but I can't find info on actually booking the tour. From the sounds of it, these aren't done often, and people actually have to book 6+ months in advance. This pdf has some good info, and photos of the tour (all in French):
http://ademas.assoc.free.fr/pdf/une_nuit_en_metro.pdf

Anyone ever done this? Can anyone find info on when the tours are offered and/or how to book?

Seems like I may want to wait for a future trip to Paris to do this, and after I've gotten more of a handle on the French language.