Now I'm changing the 'rules' which means she is looking away, ears forward and I'm not treating her because I'm trying to shape a new behaviour - ie stand on the mat
Missed this bit in your post
OK, so you've trained look away, ears forward. She probably does this when you're standing in a specific position? That's currently your cue - so as soon as you appear with treats and stand like that, she'll start offering, because you've cued her

Sometimes, we cue them just by being there (mine offers a sort of pose when I appear at the gate).
But you want to get the behaviour on a specific cue - so as above, you add this into your training loop - new cue, old cue, behaviour, click, treat, new cue, old cue etc. Until the behaviour appears consistently when you present the new cue. Then you fade the old cue (only reward when the behaviour is in response to the new cue, never when it happens at other times).
And now you want to add a new behaviour as well

You want stand on the mat? Are you near the mat when doing the look away/ears forward? If so, you start to reward for look away/ears forward/shift weight in direction of mat... You might want to get the weight shift with a slide down a lead, or just free shape it, whatever way, you're just adding a new criterion to the training.
Alternatively, take horse out of look away/ears forward set up and lead them off to a different place to do the mat training, and start from scratch

However, if you use the loopy training

as Alex describes, you can actually use the original behaviour (head away/ears forward) to reinforce the new behaviour (shift towards mat) and help the training along
Oh dear, I really do need to go catch a train now. It's just really interesting stuff I've been doing a lot of recently
