I see how this is difficult to implement UX wise. So given that this non-bumping should be limited to a diverse set of rare cases, here are two thoughts:
One way could be a temporary no-bump mode that staff could enter. Perhaps it should even have an automatic time out of a minute or so, preventing people from forgetting to turn it off.
Another one might be to not actually prevent the bump itself but make it possible to revert it via the post history modal. There could be an additional button there to Revert last timestamp change or
Revert timestamp to this revision