You can clone with
HTTPS or Subversion.
SWZ files are runtime SWF libraries signed by Adobe. The idea is that the player caches them, and if SWFs from different sites try to load the same SWZ, the player uses the signature to check that they really are loading the same file, and loads the cached file instead of retrieving it from the web server every time.
I haven't found any public documentation of the file format.
An example application that uses SWZs is this sample from Adobe http://www.adobe.com/devnet/flash/articles/vidtemplate-engage-with-flash.html
I've taken a quick look at an SWZ file from flex. The content looks random, so I suspect they may be encrypted with a key from Adobe's (which would be very bad). Or they may be just compressed in an unknown way, which would be bad, but less bad.
Luckily, most movies that use SWZs also define failover URLs that are to be loaded if loading SWZ fails. These failovers are often ordinary SWFs that Lightspark can parse. (The implementation of the failover functionality is in the flex source tree in frameworks/projects/framework/src/mx/core/CrossDomainRSLItem.as)
There was an unrelated bug that blocked the failover path from working, but that should now be fixed by e2aa85b.
Unfortunately, not all movies have failovers (or the failover is another SWZ) so this doesn't provide a completely reliable workaround. An example of a movie that requires true SWZ support is http://www.vr.fi/fi/index/aikataulut/junatkartalla.html.