Suggestion: opt-out break vs opt-in
The way Workrave prompts for break is to say that it's coming. Then, the break starts when user opts-in by stopping any input activity.
I would appreciate the option for an opt-out. I imagine the prompt would start the break unless I choose to postpone. Perhaps a simple left-click on the applet or tray icon would be required to postpone and/or a system-wide keyboard shortcut used. If I just keep doing regular activity, the break will go ahead.
This would certainly be only an extra option, not a change to Workrave's default behavior.
Workrave has something like this. You can disable "Maximum number of prompts" (https://workrave.org/docs/settings/timers/). This will force a break when the break warning window disappears. In the break window, you can postpone th break. I could somehow add an option to postpone the break during the break warning windows using the methods you describe.