At the moment, wget based mirrors of a JILS instance need to simply pull a new copy periodically, as there's no programmatic way to identify whether anything's changed without doing so.
Although it doesn't take long, it's a waste of bandwidth, especially as the mtime's of the files on disk will change, so any downstream proxies revalidating against (for example) projects.bentasker.co.uk will need to reacquire pages even if they haven't actually changed.
One solution would be to add a new "API" call, much like the status page.
Any change against an issue will result in an entry in JIRAs worklog, so returning a simple count of entries for the relevant project filter would be sufficient for a client to identify if anything's changed (though if it has, they'd still need to pull everything).
A query along the lines of
SELECT count(ja.ID) AS actioncount
FROM jiraissue AS a
LEFT JOIN project AS b on a.PROJECT = b.ID
LEFT JOIN jiraaction AS ja ON ja.issueid=a.ID
WHERE b.pkey IN ('JILS');
Would probably be sufficient