Hmm, don't think I would want to chance it, since the Fenix website lists the input voltage as only 0.9-3.3V for the the L2S head. Unless your 14500 was somewhat depleted, I'd worry that hot off the charger would damage the light.
The more expensive LxT series (which are rate up to 4.0V) seem to handle a single 14500 fine, but don't know about the other series.