Batch Enable alt text for image fields
Each content type will have a form associated with the content type. Using hook_form_alter, you can change these items and set them in batch.

Here is a great video that teaches how to do what you are asking, but using the author check box.  Using a technique like this you could set everything to 'on' (1).

http://mustardseedmedia.com/podcast/episode53

How many is 'Tonnes' - do you have 50 content types or 500? Remember you would have to write custom code to do this, so you would need to be familiar with PHP and how modules are written.  If that is not your forte and there is not a module, it may be best just to dedicate a few hours and a beer to it :)