It's not all that hard if you're willing to put a little extra burden on the level designer and have them manually create a jjRequiredFilenames list or something. AS wouldn't need to know whether a given file was music, or sprites, or a palette, or what, just that it was required for the level. The only problem is that we don't have any code to work from for sending variable numbers of files to a client -- right now servers will send at most one .j2l, at most one .j2t, at most one .j2as, and at most one (skippable) music file. Adding a non-constant number of extra files would be a bit trickier, and the thing is, tools to make custom sprites for JJ2 don't even exist right now, so why bother?