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Thanks, Ghia!!! This was exactly what I was looking for!
There was a little bit more work involved (I had to add a new array for new full post in a thread), and define "POST_REPLY_URL" as your suggested solution
{POST_URL|replace:'viewtopic':'reply'}
didn't work, but now it is working here on XOOPS, as I wanted...
The beauty of this is that the forums will work the same as a mailing list, i.e. I receive an email with the content of the new message in the thread that I subscribe to, and with one click I can respond to that message if I want to. The same as I would hit "Reply" button on an email.
So thanks again!