It's probably easier to write something in LUA to handle this. Seems pretty straightforward looking at the standard. Might be wrong though.
Although not rocket-science, I believe you might be underestimating it a bit :-)
I have not looked at the standard and have no experience at all with the protocol, but the code is in spandsp for a reason, there are some per-sample digital audio calculations, tone/silence detection etc, with strict rules/timing that most likely you won't be able to do from LUA.
See libs/spandsp/src/ademco_contactid.c comments
I believe the right approach is to write a mod_ademco wrapper that exposes the functionality to the dialplan (either through dialplan applications or functions), then you can write LUA scripts :-)