Do you know if there is any reason not to put 9v lithium long life batteries into CO alarms?

I spoke with our manufacturers and had the following answer (shortened!):

Lithium batteries are used in some CO alarms. However, the batteries specified by a manufacturer are tested and the lifespan of the alarm unit is understood with these batteries. Batteries that have not been tested or batteries which have varying quality may adversely affect the operation of the Carbon Monoxide alarm. Manufacturers must have a high degree of confidence that the batteries used are suitable for the purpose.

The Alarm unit, as part of the safety features and standards, checks the batteries for voltage. This is typically a pulse impedance method. The chemical makeup of some batteries can be unsuitable for alarm applications. Normally these types are cheaper nickel cadmium batteries where the current or voltage tail off is very sharp. This results in difficulty for the monitoring to provide sufficient end of battery warning chirp.