Here’s a userscript I just created to allow easily unchecking, checking, and setting dates for all rows on the breaks page. It adds the following controls to the top of the breaks page:
It’s pretty rough, but it works. 
Here are links to all three Beeminder userscripts I’ve created so far:
- Beeminder Power Breaks - Add power features to Beeminder breaks page.
- Beeminder Filters - Add filter buttons to your goals page.
- Beeminder Open Tabs - Add links to Beeminder dashboard for opening goals in new tabs / windows.