It would become known and be blocked. You think if the EU started rebadging and reexporting Chinese EVs to the US the EU tariff wouldn't jumt up to the Chinese level very quickly?
IANA AI researcher, but there are different technique for fingerprinting or assessing the similarity of different LLM models. Or it could become known via a leak, or good old humint.
If it wanted to, the US could make model inspection (with direct access to the weights) a condition of access to their market.