Answer
Can I request a waiver after my paper is accepted?
Usually not. Most publishers require the request at or before submission. BMJ states that applications made during peer review or after acceptance are not considered. IOP does not consider discretionary requests once review has started. Springer Nature and BMC require them at the point of submission. Cambridge requires them before the article enters editorial consideration.
Detail
Why the deadline exists
Publishers separate the fee decision from peer review. A request arriving after acceptance would sit on the wrong side of that line.
The exception
Where a waiver is applied automatically from your affiliation, no request is needed and the deadline does not arise. The risk is assuming automation at a publisher that requires an application.