Where do I found the "form action" call in TPLs to an Event?
Most / All forms inherit the form code from:

templates/CRM/Form/default.tpl

I dont think you should modify that. However, the form tag also emits all attributes attached to the form. So your best bet will be to add an attribute to the form object in the buildForm hook for event registration. There is a method call on the form object to add an attribute:

$form->updateAttributes()

lobo