Correct - would be relatively easy to write other Smartthings mods or update the existing app/device to support triggers based on what you want it to do. The plugin I wrote for the Honeywell/Ademco alarm system ([RELEASE] Honeywell / Ademco Vista 20P Integration) integrates with the SmartThings Home Monitor, allowing you to control the alarm system from the SmartThings main dashboard.
If you want it to act as a PA, I think the biggest task will be around controlling the input source. Ie. what source do you want to use and what do you want to do with it?
You can just download the zip, extract, configure and run it!