I think the simple answer is No. The NPPL is an aeroplane licence and to teach for either SEP or TMG you would need to hold an Aeroplane Licence with an associated Instructor certificate. If you can't instruct for it, you can't examine it either. From what I can see there is no requirement for a DTO to teach for the NPPL therefore, subject to having an Aeroplane Instructor Certificate, it could be conducted at a gliding club.