A long time ago I wrote a patch for LJ to add support for programmatical uploading of S2 layers to the server and an accompanying client script. Before the patch was committed, it was changed to use digest auth but the client was never updated. I've now updated it so that it can be used to upload layers to the server from the command line or as a tool in your text editor.
The upload utility is s2up and will hopefully be usable with FotoBilder too at some point. (I or someone else will have to change the server-side interface to work with FB). I personally now have it set up so that I can just hit a key in my text editor and the layer I'm working on is uploaded to the server, which is much faster than copying and pasting to the HTML form with each change.
It's a pretty simple script — it just uses the LWP library. It also serves as a reference implementation for the upload protocol, whose docs will probably be available somewhere in the future but I don't really know where to put them right now.