I had:

.htaccess

    #Allow for revisioned assets
    RewriteRule ^cdn/([0-9a-z])*/(.*) /$2 [L]

Vhost:

    <LocationMatch "/cdn(/|$)">
      Header unset ETag
      FileETag None
      ExpiresDefault "access plus 1 year"
    </LocationMatch>

The problem is the first rewrite is completed before the match in the vhost, to the LocationMatch never...well...matches? I think.