Russell wrote:You can, legally, carry on a ride along if they haven't said no otherwise.
You can 
legally carry even if they have said "no". As a government agency, they can't give binding 30.06 notice, and a police car is not off limits by statute.
If a department has a policy against CHL ride-alongs, they won't let you ride if they know you're carrying. There's no legal consequence if you do -- at worst, they won't let you ride again.
Disclaimer: police sometimes enter places that are off-limits to CHL holders, such as 51% bars and school buildings, or where you might have to face the discomfort of having to reveal yourself (such as entering a jail holding area, where your sponsoring cop also disarms). Take this into consideration.