I suspect that Chomsky would consider child porn - or at least the part of it that is illegal - not speech in this sense.

Speech is the process of communication ideas. Porn is the process of sexually arousing someone remotely. Criminalizing the latter is not criminalizing the former.

You do end up criminalizing numbers though.

There has to be a point where "criminalizing a number" doesn't really work.

A HD-DVD master key is one thing, a huge binary file is another. HD-DVD master key requires specific "numberness" to work, while a binary file can still convey similar information yet be a completely different set of numbers. if I take two pictures in rapid succession of a still life, the two pictures' jpeg data and checksum hashes aren't going to be even close to matching.

Chomsky’s position is that the burden of proof is on state violence to justify itself, and I think he would agree it’s clearly justified in that case.

However, that’s a very narrowly scoped restriction (and it’s also not clear that such distribution is a type of “speech”). A more appropriate question would be: would Chomsky support free speech for an advocacy group such as NAMBLA, or for creating mangas that depict drawings similar to CP? Those are exactly the type of speech/expression that are clearly vile, yet should still be protected from interference by state violence, from the civil-libertarian perspective.