Be it proposed:

that instead of an in-place move which reuses
generic folder names like "staging" and "staging.previous", we
composite build output using unique names like
2013-08-09_05-05-26-B7222/ or 2013-08-13_10-05-28-B7255

We therefore need:

a) to regenerate the composite site each time there's a new build
published, in order to remove the oldest and add the newest (keeping
only the Nth and N-1rst builds)

(I have a script that might already work for this, or would need
tweaking.)

b) heuristics to determine when an older (N-2, N-3, ... N-z) build is
no longer needed, perhaps simply by assuming no one needs it after
24hrs?

24 hours should be more that enough.

c) a cleanup script which can purge all but the builds which are no
more than 1 day old, keeping at all times at least two builds (N and
N-1)

(I have a script that already does this for folders like
http://download.jboss.org/jbosstools/builds/nightly/core/trunk/ but
might need to be tweaked to work for a new pattern of
staging/\${JOB_NAME}/<BUILD_ID>/ .)