I've search through the issues but couldn't find if there was a solution that was accepted into drush for Drupal 6.x.
I've written a module which goes through the process which update.php does, via drush. We use this at my work, Xelera Communication to mass update our installed site. We offer a websites made in drupal and we have our own hosting solution for Drupal 6.x.
I've tried to re-use as much as possible of the Drupal's built-in functions. I had to duplicate 2 functions and change them, they are
batch_process(). I tried to change them as little as possible, maybe in the future I'll "distill" them and remove unnecessary code.
I've added comments to added and changed lines. Some lines were deleted and som lines were commented out with an extra comment explaining why.
I'm somewhat new to Drupal and this is my first contribution to the community, or any open/free project for that matter. I am aware that the code needs more work to pass Drupal coding standards.
If I remember correctly; Moshe want's it this kind of feature to support progressive batches - correct me if I'm wrong. This module does NOT support progressive batches. I don't think it can be done with Drupal 6.x, unless the batch API is partially rewritten or a reimplmentet/rewrite in this module.
The main function,
drush_update_php_run_update(), is partially copied from Sacha Chua's blog (http://sachachua.com/wp/2008/09/16/drupal-and-drush-updating-the-databas...), which was for Drupal 5.x. I haven't asked her permission to use the code in my module, but I will contact her and/or post this issue-link in her comments. Since she has posted it on her blog I guess it is OK to use it.