Fwiw, I'd say at least half of the clauses I've been asked to sign were not included at contract time, but rather at post-contract "do HR paperwork" time. So I

Fwiw, I'd say at least half of the clauses I've been asked to sign were not included at contract time, but rather at post-contract "do HR paperwork" time. So I don't see how that could be construed to be 'part of the original offer'. Especially if you asked (in writing) if such a clause existed and received a negative / no response before signing the contract.