Hooking after that method wasn't as bad as I was thinking, so this is my current quick solution:
wire()->addHookAfter('ProcessLogin::buildLoginForm', function(HookEvent $event) {
$form = $event->return;
$form->get('login_name')->attr('autocomplete', 'off');
$form->get('login_pass')->attr('autocomplete', 'off');
$event->return = $form;
});