I'm not familiar with the hack you're talking about but I have an idea how you can do this pretty easily. One question though, when you hit the reply button, will it show you the answer then or will you have to actually reply and wait? The second would be a tad more difficult.
1) Add a field for post new topics form for the hidden field
2) Append this new field to the individual topic file in the new topic subroutine
3) In the reply subroutine add a statement to print out that last hidden field
4) Done. It'll be a little more complicated than that of course but that should be your plan of action. Other things to consider are when you edit a post for it to include the hidden text field.
