I'm sure she is a nice mare, however, she is not meant to be a broodmare. And what I may correct you on is the fact that I am familiar with HYPP, very much so.
You have a heterozygous mare which carries an autosomal dominate trait, therefore any N/N horse bred to this mare will have a 50 chance of inheritance. I'm pretty appalled that you justify it as being an okay thing for the chances are only 50/50. Have you ever seen a horse have a "fit" or die suddenly due to HYPP? I'd also like to add, that just because your horse is N/H, she still has the potential to carry HYPP characteristics and suffer the side effects of HYPP. And if this mare is bred to an H/H stallion, the offspring will for certain carry the genetic mutation.
The gene can not be diluted, therefore, the ONLY way to rid of this genetic mutation is NOT to breed N/H or H/H horses. Period. My tongue is almost bleeding for I'm trying to hold back and be as nice as possible about this.
And yes, it is a huge shame that there are impotent backyard breeders that continue to breed horses endlessly. But WHY join them in the game merely to add to your sale?
It is not worth it and breeding this mare will in no way improve future offspring or add to the breed which is the entire purpose in its self. Yes, several people believe HYPP horses to be valuable because of the nature of the gene - face it, it wins classes. But what justification is this? The saddest, I'm sure.