I didn't have the async endpoints you listed above turned on in the account I was testing this on, so I have not been able to verify this is true in those endpoints as well. By using the hub dataevents endpoint (also available as async, but requires primary keys in the DE) I was able to overwrite the date with null (well, technically I believe it is 'empty' not null, but for most use cases in SFMC this would be equivalent) by just using an empty string in the passed value.
"Name": "John Smith",
This would take the original value of:
and replace it with:
Be aware that the sync endpoint will return what was posted (e.g. the payload would be the response), but the async endpoint will return an empty string
"" regardless of success or failure. I assume this is because it is undocumented.