Internally, here's what apache is doing when mod_rewrite makes a per-dir subsutiton. This might explain a little better the sequence is
mod_rewrite internally redirects the current request to a new request w/ the substituted URL
Receive a request for /bar
Merge location/location/match config sections for registered modules
Map URL to file
Merge directory/fies/etc ...
Check access control
mod_rewrite doesn't make a substitution this time
handler (mod_php, mod_proxy_fcgi, etc) runs and starts to write output
headers get committed by the core of apache
mod_expires and mod_headers get notified when the headers are being committed and consult their configuration
Not much is retained between the initial request and the "internal redirect". One thing that is retained is the environment variables, but differentiated by the REDIRECT_ prefix.
If you are on 2.4, you could set an environment variable and check for the REDIRECT_ version and then use <if> instead of <location> to configure the expires/headers. mod_headers also directly supports expressions or env= but mod_expires does not.
