Mini Herefords do not maintain any separation in registery from the rest of the Hereford breed. So really everyone recommending smaller framed Herefords - that's what they are. There are limits to the size of the cattle being shown, though I'm not sure exactly what they are. It's definitely a niche market, but can be a very lucrative one. The other posters are right that these cattle would get beat up beyond belief at the sale barn, but if you have a marketing plan (like the locker beef you mentioned) for the lower quality stuff, the higher end one's can be very valuable for the show ring.
I know there are a few mini Herefords at some open shows, but the bigger demand for show quality ones comes from Texas' pre-junior steer program, which allows kids too young to show in 4H shows to participate (usually along with older siblings, other family or friends, etc...) in regular prospect/market steer shows, by giving them their own division to compete in. If I'm not mistaken, there are mini Hereford classes for older kids at some shows now, too. It's taking hold in Oklahoma now, and probably spreading other places as well. There's a lot of money being spent.
If you do choose to pursue it, I'd try to start out with the best show type genetics I could find. Unless you can cultivate a really good premium market for the beef, selling show prospects is the only way you can make it pay, so you need to be raising good ones.