There are some words that contracts can use that indicate an intent to disavow a reasonableness standard:
One of them is "in my sole and absolute discretion". For example, "Buyer may cease to purchase additional goods at any time under this contract in his sole and absolute discretion."
Another is "time is of the essence", which means that strict compliance with the deadlines in a contract must be observed even if it would seem to be unreasonable to insist upon them.
Another would be to have a term that states something like: "Due to the fact that it could pose an extreme danger to end users of this product if it does not exactly meet every specification set forth in this contract, strict compliance with every specification of this contract shall be required and manufacturer shall not be entitled to any compensation under this contract if the Widgets produced hereunder have even slight or accidental defects even if the manufacturer has substantially performed the work under this contract."
The ability to waive an implied reasonableness term usually goes hand in hand with the contract doctrine that holds "penalties" in contracts to be void as against public policy, but upholds "liquidated damages" clauses.
What this means in the context of reasonableness is that if the consequences of not requiring someone to be reasonable in a contract leads the other party to forfeit a right to benefit under the contract that is grossly disproportionate to the foreseeable harm caused if a reasonableness term is implied in law, then the waiver of a reasonableness requirement is probably not valid because it is a de facto penalty and not merely compensation for the harm caused by the de minimus breach of the contract, perhaps due to the unreasonableness of the other party.
But, if the exact harm to the other party from impairing their absolute discretion or ignoring a strict reading of the contract and instead allowing an implied reasonableness term to color the meaning of the contract is material but is hard to quantify, and the consequences to the breaching party are crudely proportionate to that hard to quantify harm, then a waiver of the implied duty to be reasonable will usually be upheld as valid, as the consequences of not allowing reasonableness do not extend beyond the compensatory relief normally allowed in a contract.