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A quick and dirty hack that might do what you need: export your entire database to .archive format. Close OmniFocus. Double-click on the exported file, which will launch OmniFocus and only open that data. Now select the entire database, bring up the inspector, set start date to today. Now clear the start date field in the inspector and tab to a different field. This should erase every start date in the document. Rejoin your previous workflow with no start dates in sight.

Quit and relaunch OmniFocus to get back to your usual database.