Okay so I was looking through some old txt files, from 2004, and in one of them I found this note that I made to myself:

"1138 ENCORE torrent"

I am not sure if I made the note in 2003 or 2004, because it was in my 2004 folder, but it does confirm a few things I remember.

I just did a Google search and found this link:

http://boards.theforce.net/threads/thx-1138.7175997/

People in there stated multiple times that they caught THX on Encore. While their posts date 2002, and Encore did not start airing HD until March 22, 2003, I firmly believe that it was this channel that had THX in HD.

With that said, I can't imagine if it was aired in HD, and someone actually capped it, that it exists now. But I am going to email Encore to find out if they ever aired it in HD.