I figured if you're smart enough to use a snapshot/prerelease, you're smart enough to move a few files around.  There's a folder named "Snapshot" in the zip file, go into it, copy all the files onto your computer, go to the texture pack zip file, and then copy the files you copied onto your computer into the texture pack zip file directly.  If it asks you to replace the files, make sure you say yes to any and all of those options.  Now the texture pack is ready for the newest Snapshot/prerelease.  Have fun!  :D

Note: If there are no files in the snapshot folder, then I have not updated yet.