It is very difficult if you don't have an "in" with an agency. To departments it does seem strange to have a random dealer approach them about NFA stuff. The agencies around here are generally getting rid of their FA stuff in favor of semi, they just don't have the training to stay proficient. Plus the added scrutiny of LE in this day and age doesn't help either.
I'd recommend contacting their firearms staff before the command staff, if you can find a "gun guy" and explain what it is you're offering (the demo/no obligation to buy etc) that person can sometimes smooth the path to getting a letter from the Chief. That's worked for me in the past. Sometimes you have to sell them other products to get your foot in the door, even at cost just to establish that relationship.
Good luck!