What a stupid idea! We already have a file system that stores, wait for it..., FILES. Why create a new format that stores files within files? I can quite easily reproduce this "feature" with a two-line shell script:
exec $0.`uname -m`
Put the binaries for each architecture in the same directory (with the arch as a filename suffix), link this script to the name of each binary (without the suffix) and you are done.
Or, you could easily add a feature to the packaging system to install the proper binary for the correct architecture and not waste disk space on other unused arch binaries.