I have no idea about "etiquette", but if you were stopped, common sense should have told her to go around you.  It was her fault she hit you, even if your mare did move a bit.  If you think about it, if you mare hadn't moved her hip, the other horse still would have passed extremely close to you and crowded you on the rail.  She was expecting you to move for her which seems a bit arrogant to me.

 I don't show, but I always thought that everyone was supposed to be going in the same direction. 

There really is no such thing!  LOL.  That little hussy was pulling one over on you, BIG TIME!  If you had stopped then she should have moved around you.  I might have grown some devil horns, hunted her tail down and trashed her arena time.  I absolutely would have stuffed my spur into the side of her stirrup or raked it down her horse.  It's quite amazing how well animals react to "discomforts."  Having the ends of the reins ready to whip intruders can also help define your space.  As people approach loudly stating your horse (insert any aggressive behavior, I use kicks/bites) will usually cause others to give you some room.  Riding along with your bat/crop stuck out also works REALLY well.

Here's an example of some tactics I've used:

I had another contestant ruin one of my HUS runs once, pulled right into the rail after passing me and stopped.  I took it upon myself to "help her learn from her mistake" and promptly drove her and her little junker into the wall on the next upcoming corner.  Later her mother approached mine at our stalls.  She stated that her daughter hadn't done anything on purpose but wasn't a good rider and that, "things happen in the ring."  My mother replied with, "yes they do."  The lady stomped off without the apology she thought was coming.  As she reached the end of the stall row my mother loudly stated, "  My daughter isn't a good rider either, she's great!"  The next class up, HSE, I took second out of 71.  The lady never knew I was standing in the tack room listening and adjusting my helmet.

Once my students were ready to show physically I started training them mentally.  Thinking ahead can sometimes be more than youth competitors can handle, it's also where the worst wrecks could happen.  I would give "bumper car" lessons.  I'd mount up and we'd pass each other, learn to think about up coming transitions, corners, spacing and avoiding reckless riders.  Once they could handle the situations I put them in, still show the horse and maintain fairly good position they were ready to compete.

I always try and avoid situations that could lead to a wreck even if it means I stop. Who cares who was at fault when it can lead to a injury to rider or horse or ruining the rest of the day? If someone is that intent on having the rail, they can go ahead and have it. You can always call out your intentions- RAIL.....INSIDE....OUTSIDE>>>>>something to let them know you intend on turning in or out.

I also try to ride in the general direction of the crowd to avoid problems. If the ring is light, then I will ride in any direction, if traffic is heavy, I'll go with the flow. IMO- proper ring etiquette means making it safe for yourself as well as being polite to others..........

Rules I was taught -

pass left to left except:

-the rider doing lateral work along the wall always has the right of way

-those riding on a circle always ride inside of those on the track

-do not halt on the track, move to the "second track" to halt so traffic can pass to your outside.

-also move to the second track if performing difficult moves such as tempi changes.

-those just coming off a diagonal always yield to those on the track and therefore, if necessary pass inside even if right to right.

-do not under any circumstances run over your instructor.

-if you fall off in the ring, you must buy the entire class a beer afterwards.

Thanks for bringing up a great topic.  I have often wondered the same thing. My way thus far has been to circle (if I am coming up behind someone) or to shout something like "I'll take the inside... (or the rail)."

Our riding club recently constructed a warm up ring at the fairground where we hold our shows. I think maybe i will post some "suggested maneuvers" to avoid collisions.

Rodeo warm-up...enough said.

Visual images of this have me almost peeing my pants. 

There are no rules, just survival strategies.