cowboy: session state and connection timeout

i'm building a web app using cowboy with mnesia as persistent storage.
the current version uses http (may move to websockets in the future).

during the lifetime of a user's session, i need to maintain per-user 
state that is available to all route handlers.
i plan to initialize this state when a user logs in and write parts of 
it to persistent storage when the session ends.
sessions can terminate because of the user logging out or because of a 
connection timeout.

if a user logs out, then my javascript can hit a route which can write 
persistent storage.
if the connection times out, i need to get a message (presumably from 
cowboy or ranch) to one of my processes so that it can write persistent 
storage.

i scoured the documentation for cowboy and ranch but didn't find any 
mention of a mechanism to receive a message on connection timeout.

any help would be appreciated.
thanx.