Sure, in the explicit validate block. All variables in the form are available at that point. The variable you give as the reference variable only serves to tell ad_form where to put the error message if the check isn't passed, it plays no role at all in the actual checking.

So in your example you'd probably do something like

-validate {
  { password_again
     { [string equal $password $password_again] }
     "learn to type, Dave!"
  }
}