With only a little bit of searching, I found quite a few who are doing very well!
Here's one of my favorites:
Meet Quatro Clabber, a 2006 AQHA Stallion currently competing in 1st Level on the east coast:
Seriously, how cute is he?
He is also a licensed AWR Approved Stallion for Dressage. Not bad for a short little stock horse!
I'm thinking that every week or so I'll spotlight a stock horse competing in dressage or other "nontraditional" discipline (jumping, eventing, etc). Mostly because I'm just curious to learn more about them! If any of you know of a horse I should hear about, post it in a comment :)
At any rate, it gives me hope for me and Penny in a few years. She's such a cute mare <3