Having just purchased this same pistol for hunting in thick cover where animals tend to retreat to,
I would think a brace would be ok to hunt with.
I have a Knight Bighorn which I have hunted with for years and always found it heavy and cumbersome. (Insert old age here) The Optima v2 reduced the weight to about 3 lbs, the Knight is 8 + lbs, both preform great in their own ways. The pistol reduces weight giving me more abilities to move with more stealth by approaching animals by crawling if need be. A long gun is difficult at 8 lbs lbs, however hunting in Eastern Wa. sage country for mule deer it was doable but awkward.
Presently I am carrying the pistol in a cross body chest holster which frees up my hands for grasping trees/brush as needed to move silently with the ability to crawl, in addition I will carry shooting sticks.
Without knowing your purpose for a brace be it hunting or whatever it is something only you can decide, I see nothing in the game regulations stating you couldn't hunt a muzzleloader pistol with a brace.
The regulations are caliber specific for pistols with strict guidelines as to caliber and barrel length and loaded from the front of the barrel, double barrels are limited to one barrel loaded only.
Check the regulations to be sure as I may have missed something 😅 as the regulations are quite wordy and one needs a lawyer friend to hunt with before pulling the trigger.🤣
If you are planning to hunt with your braced pistol I would love to hear about your experiences.