So to explain how I unpacked the scene.pck file, a while back I came across a project on GitHub that contained tools to work with Rewrite. Pretty much, the tools already existed, it was a matter of figuring out what key is used to obfuscate the gameexe.dat and scene.pck files. I made a fork of the project with the patches made to work with AB 1st beat trial ed: https://github.com/bitprime/vn_translation_tools/tree/master/rewrite
Here’s a link to the tools already built for those without a compiler: https://dl.dropboxusercontent.com/u/58805276/AngelBeats_1stBeat_Trial_Tools.zip
Basically, use unpackscene.bat to extract out scene.pck. Then use packscene-compressed.bat to rebuild the scene file. I did a quick test and it seems to work just fine, but let me know if you guys encounter any problems.
When the final version of Angel Beats comes out, it’ll most likely have a different key used to encrypt the game/scene data, as it seems every other game that uses Siglus had a different key. In that case, the tools will have to be updated with the new key.