Yes, in theory, we've been through this rodeo before in the 90s. Bernstein vs. US ruled that source code is expressive speech. It's being challenged though, the court could consider weights as machine data (non-expressive operational artifacts, not human communication).

the US could also pull the functional harm argument in the same way the government uses it to regulate some protected speech (like the whole debacle over DeCSS)

Even if the free speech argument prevails, the US GOV can still use sanctions to prevent the public clouds from hosting any foreign open weight models, and prohibit US based sites from distributing the weights. It'll remain functionally legal for individuals (or at least, unenforceable) but in practice, you'll need to pirate the weights they won't be available from any US based site or provider and would be illegal for businesses to use, even if its a gray area, the liability will mean businesses won't touch them.