How to dynamically assign submit hooks to webforms when generating blocks
webform_block_view() returns a block object that already has the rendered webform. At that point, you're not going to be able to make any form altercations need for your custom submission handlers.  

However, you can add a hook_form_alter() invocation to your custom module. Webform generates a standard form id for each form webform_client_form_NID. 

MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if($form_id == 'webform_client_form_NID') { //@TODO: substitute NID for your webform NID
    $form['#submit'][] = 'MYMODULE_webform_client_form_NID_submit';
  }
}


If you need to pass data from your hook_block_view invocation to your hook_form_alter invocation, a global variable should help there.