Hmmm...
I assume private organizations are out, so no Gatekeepers, Hellsing, Full Metal Panic, or XMen type ones. Stargate is military, as is Robotech/Macross and XCOM.
Acts as one while looking like Nene in:
R-AA 'Crossed Worlds Bubblegum Crisis' by Multiple - [Status=Not Complete] - [Last Dld=07/10/2008] - [Size=105kb]
R-AA 'Hyper Police 12' by Multiple - [Status=Not Complete] - [Last Dld=02/14/2004] - [Size=64kb]
R-AA 'Ranma GXP' by Multiple - [Status=Not Complete] - [Last Dld=06/07/2008] - [Size=403kb]
R-AB 'Twilights End' by radical edward9 - [Status=Not Complete] - [Last Dld=07/18/2006] - [Size=97kb]
- Link withheld due to content
He is with the Britsh Government as a agent and trainer in:
R-MULTI 'Changing Path' by Xerxez - [Status=Not Complete] - [Last Dld=09/20/2005] - [Size=412kb]
Works with them in:
R-MULTI 'Soul's Destiny Blood's Inheritance' by kokatsu - [Status=Complete] - [Last Dld=11/03/2004] - [Size=505kb]
Commissioned by them in the 1800's in:
R-RK 'A Ghost of Tomorrow' by Proteus - [Status=Not Complete] - [Last Dld=11/07/2003] - [Size=385kb]
Contracted/blackmailed by city to hunt and bring in Sailor Senshi:
R-SM 'Vigilantes' by Michael Fetter - [Status=Single File] - [Last Dld=08/04/2003] - [Size=4kb]
RANMA 'Red Eyes Fury' by Animeprime - [Status=Complete] - [Last Dld=03/16/2004] - [Size=79kb]
A runner up, with him working with the legal system, would be:
RANMA 'The Art of Justice' by Ms Spooky1 - [Status=Not Complete] - [Last Dld=06/13/2006] - [Size=63kb]
Another runner up would be:
RANMA 'To Tell a Hawk From a Handsaw' by N Reynolds - [Status=Complete] - [Last Dld=07/29/2004] - [Size=188kb]
A final runner up would be:
RANMA 'Thyself Known' by Zorknot - [Status=Not Complete] - [Last Dld=10/06/2007] - [Size=1079kb]
EDIT:
I think he was a cop in:
RANMA'2 Ways to Teach' by independen-and-happy1373 - [Status=Not Complete] - [Last Dld=06/22/2004] - [Size=107kb]