The podcast was a bit confusing. The best policy regarding the Rulebook is to read it for yourself. If you still have a question, call or email the USEF or check with the TD at the show before you mount up. The salient pages in the Rulebook are easy to download from the USEF website. (Note, that is USEF, not USDF. USEF writes the rules, not USDF.) As a good plan, print out DR 120-124, read them before you start to show this year, and stash them in your show kit for quick reference at a show. (You can also quickly access the USEF Rulebook on your cell phone at a show, if need be.) This is especially true this year since there are so many new rules, which are highlighted in red.
DR 120.2.1 is simple and clear. Yes, polos are fine and should be open with the collar in the usual place. (Keep an appropriate polo in your show kit.) A rule change further down in the “elimination” section may have greater impact. Judges used to have some discretion in eliminating someone for a dress violation, but that is no longer the case. The reasoning is that riders who weren’t eliminated earlier in the year for a dress violation then maybe made the same mistake later (ie in Championships) and were eliminated at a costlier show. Bottom line: Remove your stock tie if you remove your jacket.