You can shoot in IDPA in the NSF division. Do it, it is fun!
The only place NSF appears in the rule book is in section 2.2.D.g "while transferring a firearm from one hand to the other."
I think I found it, SPD.
8.2.10 Specialty Divisions (SPD)
A. IDPA encourages shooters to practice their gun handling skills with commonly carried firearms. Many everyday carry firearms do not fit into the 6 competition handgun divisions.
B. IDPA allows clubs to add “Specialty Divisions” for scoring. This division allows cartridges smaller than 9 mm, carry optics, activated lasers, mounted lights, and other firearms which do not fit into the other competition divisions to participate in matches.
C. All other IDPA equipment rules apply for holsters and loading device holders as well as their placement on the body. Match Directors also have the option to allow junior shooters with .22 rimfire firearms to begin strings at low ready in lieu of requiring a holster.
D. All IDPA membership rules apply.
E. Specialty Division classifications are based on the shooters highest classification of regular IPDA divisions.
F. Clubs are not required to implement this provision, and Match Directors are allowed discretion with implementation so that match quality remains high.