Antimalware engines have already had to support unpacking somewhat arbitrary things, sometimes with serious comedic effects when people fuzzed those unpackers; I don't see this significantly increasing the burden of their examination, signed binaries are no more dangerous after this than before, and unsigned binaries could already bring their own arbitrary unpacker.

(At least quickly thinking about it; I could easily have overlooked some complexity, but barring an unpatched libarchive exploit, I don't think this markedly increases attack surface...and at that point, you could probably make similar arguments about adding any new file format?)