Specifically, it works by taking the stub EXE and sticking all the files to bundle as "resources" to a copy of that EXE. I'm pretty sure it's uncompressed, too!

Instead of using the resource fork as an archive, you could implement it as a tarball concatenated after the stub, as with a typical self-extracting archive. Then you would not need those exotic functions, and the source for the stub would be portable (as opposed to using OS-specific features to read the resources).

—John