Yeah, I don't think the old loophole of running GPL'ed code on your server would apply here since DOSBox would be downloaded and executed in the browser. He has to release the source.
Furthermore, he didn't just adapt the build scripts. The game files are not mounted from a discreet disk image, he made a hack to get them available from the DOSBox internal Z: drive (which isn't actually a disk image but the interface to internal functions).