As a moderator of this forum, I'm pretty familiar with what it means to post on a public forum. So yes, you're entitled to post your opinion, complaint, rant, whine, etc., and nothing you posted is anywhere close to violating a forum guideline. Similiarly, any other member is entitled to respond to your post in kind and express their opinion, as long as they stay within the forum guidelines. Not everyone who posts in response will support or agree with your opinion, and that's okay.
I will state this one time..the point of this thread was to see if it's happened anywhere else, how it was handled, opinions on this happening and how you feel about it.
However, rather than just boldly state my opinion in response, let me try to answer you more specifically.
Yes, it happens all the time, everywhere. Professionals showing as amateurs, phony ponies showing in the wrong height division, people showing "down" a division, ie, some who has won multiple equitiation classes elsewhere showing in a Maiden or Limit class, or in your case a Long Stirrup class.
Yes, I think it's unethical and unsportmanlike. After showing for decades, I guess I'm a little more jaded than you are. I have seen so much worse than this in competition it doesn't arouse much of a passionate response in me.
As far as how it was handled, sorry, the options are still exactly what I told you. If you want any action, you will have to file a protest with the show organization. If it's a USEF rated show, or run according to USEF rules, you have to put up money, usually $200., to file a protest. If your protest is sustanstiated, you get the money back. This is to prevent nuisance or "tattle" protesting. I do not know it you can do so anonymously, I know if you protest someone's amatuer status or the height of a pony, your name is attached to it, and that has the unfortunate effect of keeping people from protesting some pretty flagrant stuff.
So I wish you luck, I hope the cheating competitor is reported and prevented from doing this again. If I had students in the same class at the correct level of experience for the division, I'd be pretty ticked off.
Here's a link to the rulebook for your association - Rules
A brief glance at them shows the process for filing greivance (It's under Section 3, Hearing Committee) and yes, you can file one if you believe a competitor is ineligible to compete. There's no mention of a fee to do so, so maybe that doesn't apply to all affiliate or local show organizations.