In cases where there are two definable audiences it can make sense to adopt an audience-specific user interface scheme. This could simply be accomplished through radio buttons positioned side-by-side in the interface.
The "Authors" radio button is selected by default, With that selected the vailidator would function as it always has in the past with missing alt reported as an error.
If the "Generator Developers" radio button is selected an alert could fire.
If the user selects "OK", the new attribute would kick in and allow the page to pass validation.