Are any of those alpacas halter broke? If so, have someone lead one of them, in the arena, door closed, and let your horse follow it.
Horses get a lot of confidence, if their object of concern, seems to yield to them
First time I took Charlie to one of the local shows, there were driving classes, and mini horses. That combo put the fear of God into her, so she was ready to 'leave the country, as I was trying to warm her up outside, and where driving hroses would come up behind her.
I then asked one lady, if I could just follow behind her cart. It helped, as Charlie saw herself as moving that object of concern
When I had my reining mare down at Spruce Meadows, for demos, during the Masters, I had to ride in the evening parade, that went through the International ring. I was placed right behind a woman leading a lama.
The woman told me'go ahead and let your hrose have a sniff', when my mare seemed a bit concerned.
I gave San Stone a loose rein, and reeled her in, just in time, as she was going to take a hunk out of that beast!
How about a stall? Do you have good solid stalls at those facilities, and can you put the alpaca in a stall across from your mare?
No, flooding might not be the answer, as in throwing them in together, but at the same time, you are allowing her just to fuel her own irrational fear and doing nothing, far as solidifying any trust in you and your judgement.
If you take her near those alpacas, you must first be sure that you will remain calm, give her that leadership security, and also keep control of her, otherwise, you are obtaining 'negative training, which is worse then no training, far as accepting those beasts
We considered that as well. We do have access to secure stalls, but we didn't know how well it would work. She had an injury to her leg over a year ago that caused her to be stalled for 3 months straight. The vet didn't want her to walked much, so we were only allowed to bring her out to clean her stall or to give her a 15 minute period of grazing outside. From being stuck inside a stall for long, she went stir crazy. She got so anxious, pacing and pounding at the door to leave. Ever since then she doesn't like stalls much. We can tack her up in a stall, but for an extended period of time or overnight she doesn't like.
So we thought combining something she doesn't like, alpacas, with something else she doesn't like, being stalled, wouldn't really produce positive results. But who knows, it might be worth a try, since everything else has failed.