When replacing batteries in a power sounder I have fitted these...
...externally.
Easy to test and replace and if they leak they won't leak on the PCB!
Although I can see the logic in what you are doing, sadly that would not be sufficient to satisfy the insurance company with regards to keeping the alarm within spec.
Basically, and in laymans terms, the alarm powersounder criteria is that the siren must have an internal and uninterruptable power supply that is capable of sustaining the alarm sounder for a period of not less than five minutes, and that during that five minutes of operation, it should not be possible to deactivate the siren with or without the use of tools as part of its attack test specification.
But I must confess to really liking the idea of using something a bit beefier, plug in, commercially available and of a design not so prone to failure. How about something like small telephone or radio control car batteries fitted inside the sounder on a similar plug type arrangement? Food for thought?
:-?