In my opinion TASO is wrong here. I will follow what Steve Shaw has said and that is the back must be catching a piece of the tackle (or second lineman from the snapper).
As good of a person as Steve Shaw may be (never met him, so I have no personal knowledge), he doesn't speak FOR TASO or the UIL. TASO listens to Shaw, and assimilates his interpretations into the UIL environment as appropriate for UIL football. In this instance, there is a HUGE difference between UIL football rule exceptions and the NCAA rules. The NCAA does not have a "second lineman from the snapper" rule, therefore, when Shaw is talking about catching a piece of the tackle, he is referencing a tackle - as we know it by conventional nomenclature, not by NCAA rule - in a 'normal' tackle position (which would be the second position from the snapper). If you watched/listened to BOTH of Shaw's presentations to TASO in the past several months, he was clear that the tackle must be in a reasonably normal position, with a reasonably normal split from the first position from the snapper. He acknowledged that wide splits might put the tackle far enough from the snapper to CLEARLY be more than 5 yards from the snapper, thus, putting himself outside the tackle box. Because it is virtually NEVER going to happen in NCAA football, he did not address exceptionally narrow splits, where a third lineman from the snapper might actually be completely within 5 yards of the snapper, thus, placing the 'tackle' much too far inside the tackle box to be useful as a landmark for the limit of the tackle box. Please note that "catching a piece of the tackle" is not to be found anywhere in the actual rule language. That is nothing but a concept of convenience, based on typical, usual, normal, conventional actions of the teams - simply an aid for us to determine the legality of a back with respect to his blocking below the waist.
What I do know about Shaw is that he is a very "anti-what if" kind of guy. So, when he is addressing rules, he is thinking about typical, usual, normal, conventional things that happen, and NOT those things that COULD happen. Sadly, it is those things that COULD happen that cause controversy, because, every once in a while they DO happen. As loathe as Shaw may be to write rules to cover everything that could happen, eventually, that is what happens, because those "what ifs" raise their heads, then a team loses a game over an ambiguous rule and raises a ruckus about it, and we get a rule change to cover that twice-in-a career "what if" event.
My point is that Shaw - being in, of, and for the NCAA environment - doesn't think of the possibilities surrounding High School or Junior High football. Not that he should. But, the UIL, with TASO assistance, recognizes those possibilities, and adjusts the rules to better suit the sub-college game (at least as they see it). Thus, we have the "maximum second lineman from the snapper" rule, with regard to which lineman are allowed to BBW. That does not affect backs. The Tackle Box is still 5-yards from the snapper, and backs must catch a piece of the tackle box to be allowed to BBW (at all, and even then, only 10-2, in the box, and while the ball is within the box). Don't be technical on the 5-yards. Use a normally positioned tackle to set the limit of the TB. But, abnormally positioned tackles are then ignored, and we just have to estimate the limits of the TB.
The bulletin that was issued by TASO is our official guidance. As a member of TASO, if you are working UIL football, you must use the official guidance offered. You do NOT get to ignore that guidance, or a rule, just because you may not like it, or think it conflicts with NCAA rules. If you want something changed, follow the chain of command, and suggest a change. If you don't know the chain of command, talk to your chapter president, or your District Director.
Good luck, but do it right.