I don't do dressage at all so couldn't comment on a hackamore's suitability for advanced dressage - I will bow down to Lgd's superior knowledge there!
However, it is certainly true that a hackamore can be extremely severe - it can put tremendous pressure on the nose and also the poll. Please don't think it's like riding in a headcollar or halter. All bitless bridles of the hackamore family are designed to use pressure on the nose and poll.
My stallion was (and still is) ridden in a hackamore following an accident out on loan that left him with a near phobic reaction to bits. It's important to have an understanding of how a hackamore works before you just put one on, so I'd recommend at least one lesson for horse and rider to get used to the idea. Also bear in mind that while some horses love them and go really well in them, it's not the case that every horse does - just as some horses like snaffles and others like curbs.
A hackamore is capable of exerting a huge amount of leverage on a horse's face. Never underestimate the sensitivity of the horse's reaction to it. Think of a pressure halter, and how effective they are - and then think on to the fact that a hackamore is stronger still.
Any horse in a hackamore should still be ridden "on the bit" - you don't really need a bit for this to happen! It simply means that the horse is carrying himself correctly, engaging his quarters and working in the correct outline. My stallion even worked with a wet mouth whilst wearing his hackamore.
A hackamore must also be fitted correctly - if not, then it will interfere with the horse's breathing and can cause immense discomfort.
If you go ahead with a hackamore for Mysy, get your instructor to fit it with you and have a couple of lessons to show you how to ride with it. It's important to understand its action, as with any piece of tack.
Hope it goes well!