Show custom page while the server processes a form Form submit handler sends some time consuming requests to other servers. I need to show custom page with some simple content like timer/progress-bar/images while server will process submitted data.

Not sure how this could be properly done in Drupal 6.26.

Found useful batch tool (set_batch()), but didn't figure out how to replace default progress-bar with custom page and content.
Checkout the module loader , it may solve your problem 

A similar module hide_submit