I think concerns of abuse will always be there. Having a firm policy, standards on security, and who has access to the N20 cylinders - along with zero tolerance will be critical. Perhaps some others that are currently using N20 can chime in with what they do - or suggestions?
Unfortunately with N20 cylinders there is no way to determine what is in the cylinder by means of cylinder pressure. The gas is in a liquid form - similar to propane - and turns to a gas as it is released. This means the cylinder regulator will always read full - until it is just about empty - and the pressure will drop.
With all of the logisitcal issues regarding inhalents, why not just add IV morphine or fent?
It costs less, easier to account, and is so easy, even the drug abusers on the street can do it.
(disclaimer, I do not believe BLS should be doing pain management with anything other than NSAIDs, if people really value pain management, they need to come up with the money for local ALS, but I offer the above in an effort to solve the problems with nitrous)