You could try putting in any number of "tick box fields" (hidden) (such as "Approved", "Rejected", or whatever) that get marked when the user clicks the buttons in the form (via rules attached to the buttons).
Then, in sharepoint designer, make a workflow that sets the approval value to "approved" when the value of the 'approved' tick box in the form is changed; similarly, have the approval value change to "declined" if the "declined" tick box in the form is changed,
etc, etc, etc.
Would this work for you?