RSCDS provides two ways to make Free/Busy information available.
CalDAV REPORT Method
CalDAV specifies a particular query type to retrieve free/busy information, and from 0.6.0 this is supported.
Since no clients have yet been observed to make CalDAV free-busy-query requests, but Evolution (at least) provides an alternative based around using e-mail addresses, the free/busy information is also available on a direct request. The URL may follow the form identical to the structure for the 'caldav.php' URL, and will return freebusy information to a suitably authorised user.